• Analyzed existing codebase and duplicated assembly resolver implementations
• Identified the three locations with duplicated VS discovery logic:
  • vsintegration/tests/UnitTests/AssemblyResolver.fs
  • vsintegration/tests/FSharp.Editor.Tests/Helpers/AssemblyResolver.fs
  • vsintegration/tests/Salsa/VsMocks.fs (via vsInstallDir logic)
• REMOVED entire TestHelpers folder and moved implementation to tests/FSharp.Test.Utilities/VSInstallDiscovery.fs
• REMOVED FSHARP_VS_INSTALL_DIR environment variable and all references to it
• REMOVED testEnvironmentVariables function (marked as useless)
• ADDED VS180COMNTOOLS support for Visual Studio 2026 in tryVSCommonTools
• FIXED namespace conflict by using FSharp.Test.VSInstallDiscovery instead of FSharp.Test.Utilities.VSInstallDiscovery
• FIXED code formatting issues in AssemblyResolver.fs files
• FIXED F# indentation syntax errors in VSInstallDiscovery.fs module
• FIXED FS1204 compiler error by removing CompilerMessage attribute
• REVERTED Salsa project changes to avoid Microsoft.CodeAnalysis dependency conflicts
• REDUCED Salsa project package references to match main branch (removed 11 unnecessary packages)
• Implement graceful fallback mechanism with correct precedence:
  1. VSAPPIDDIR (derive parent of Common7/IDE)
  2. Highest version among VS*COMNTOOLS environment variables (VS180, VS170, VS160, VS150, VS140, VS120)
  3. vswhere.exe (-latest -products * -requires Microsoft.Component.MSBuild -property installationPath)
• Add proper logging and graceful degradation
• Update TWO of the three assembly resolver files to use the new centralized discovery (UnitTests and FSharp.Editor.Tests)
• Salsa project keeps its inline implementation due to dependency incompatibilities
• Added InternalsVisibleTo entries for FSharp.Editor.Tests
• Verified no references to removed functionality remain
• Code formatted according to project standards

Build Fixes Applied

Fixed namespace conflict: Changed module declaration from FSharp.Test.Utilities.VSInstallDiscovery to FSharp.Test.VSInstallDiscovery to avoid conflict with existing Utilities module in Utilities.fs.

Fixed formatting: Applied fantomas formatting to all modified files to resolve code formatting issues.

Fixed indentation syntax errors: Properly indented all content within the VSInstallDiscovery module to resolve FS0010 "Incomplete structured construct" and FS0058 "Unexpected syntax or possible incorrect indentation" errors.

Fixed FS1204 compiler error: Removed the [<CompilerMessage("This module is for test code only", 1204, IsHidden=true)>] attribute that was causing compilation errors in test projects.

Fixed Microsoft.CodeAnalysis version conflicts: Reverted changes to Salsa project (VsMocks.fs and VisualFSharp.Salsa.fsproj) to avoid dependency conflicts. The Salsa project has incompatible dependencies with FSharp.Test.Utilities due to different Microsoft.CodeAnalysis versions, so it keeps its original inline VS discovery implementation.

Reduced package references: Aligned Salsa project package references with main branch, removing 11 unnecessary packages that were inadvertently added:

• Microsoft.VisualStudio.Interop
• Microsoft.VisualStudio.GraphModel
• Microsoft.VisualStudio.ImageCatalog
• Microsoft.VisualStudio.ProjectAggregator
• Microsoft.VisualStudio.Shell.15.0
• Microsoft.VisualStudio.Shell.Framework
• Microsoft.VisualStudio.Text.Data
• Microsoft.VisualStudio.Text.Internal
• Microsoft.VisualStudio.Utilities
• Microsoft.VisualStudio.Validation
• Newtonsoft.Json

Updated references: Two assembly resolver files (UnitTests and FSharp.Editor.Tests) now correctly reference FSharp.Test.VSInstallDiscovery module. Salsa remains unchanged to avoid dependency issues.

The implementation addresses all build failures:

• ✅ Resolved FS0247 namespace/module conflict
• ✅ Resolved FS0039 value not defined error in VsMocks.fs
• ✅ Resolved FS0010 incomplete structured construct errors
• ✅ Resolved FS0058 indentation/offside rule errors
• ✅ Resolved FS1204 compiler message error
• ✅ Resolved MSB3277 Microsoft.CodeAnalysis version conflict errors
• ✅ Reduced unnecessary package references to match main branch
• ✅ Fixed code formatting issues that were causing CI failures

This pull request was created as a result of the following prompt from Copilot chat.

Implement test-only Visual Studio installation discovery infrastructure, based on the (now closed) prior draft PR #18900 (Implement robust Visual Studio installation discovery for integration/editor tests), but limiting all changes strictly to test code. Goal: provide a centralized, robust, and graceful discovery mechanism for Visual Studio installations used by integration/editor/unit tests under vsintegration/tests while avoiding any changes to non-test (product) code.

Context:
The previous PR #18900 introduced a helper (VSInstallDiscovery.fs) and refactored three duplicated assembly resolver implementations in:

• vsintegration/tests/UnitTests/AssemblyResolver.fs
• vsintegration/tests/FSharp.Editor.Tests/Helpers/AssemblyResolver.fs
• vsintegration/tests/Salsa/VsMocks.fs

It consolidated logic to discover a VS installation via precedence:

1. FSHARP_VS_INSTALL_DIR (explicit override)
2. VSAPPIDDIR (derive parent of Common7/IDE)
3. Highest version among environment variables matching VS*COMNTOOLS (e.g., VS170COMNTOOLS, VS160COMNTOOLS)
4. vswhere.exe (-latest -products * -requires Microsoft.Component.MSBuild -property installationPath)

Problems to solve (tests currently):

• Hard fail (failwith) when VS170COMNTOOLS or VSAPPIDDIR not present, preventing partial test execution.
• Duplicated logic scattered in multiple resolver files.
• No graceful degradation / logging when VS absent.

Requirements for this new PR:

• Reintroduce (or create if absent) vsintegration/tests/TestHelpers/VSInstallDiscovery.fs containing the discovery logic; mark with [] module for convenient access.
• Expose:
  • let HasVisualStudio : bool
  • let VSRoot : string (empty if none)
  • let getVSProbingPaths : unit -> string list (existing directories for assembly probing)
  • let addVSAssemblyResolver : unit -> System.IDisposable (registers an AssemblyResolve handler only if probing paths exist; returns noop disposable otherwise)
• Refactor the three resolver/VS mock related files to remove duplicated environment scanning. Each should simply call addVSAssemblyResolver (or conditionally build mocks only if HasVisualStudio) and log a single line if VS not found.
• Logging: Use a consistent prefix "[FSharp Tests]"; e.g., "[FSharp Tests] No Visual Studio installation found; proceeding with reduced functionality." Avoid throwing.
• Provide graceful absence: tests that do not require VS continue running; only those that actually depend on VS assemblies may skip or yield limited behavior (do not add global skips unless already present infrastructure exists).
• Ensure no modifications outside test directories (vsintegration/tests/**). Do not touch compiler/product source.
• Keep changes minimal: focus on eliminating failwith calls and duplication.
• Include comments summarizing precedence and rationale at top of VSInstallDiscovery.fs.
• Add simple unit test (if feasible within existing test framework) validating precedence ordering (can simulate environment variables by temporarily setting process-level environment variables) and that absence leads to HasVisualStudio=false without exception.
• Ensure cross-platform safety: code should early-exit gracefully on non-Windows (most VS-specific logic relevant only on Windows). Guard Windows-specific filesystem patterns, vswhere invocation, and environment variable parsing with runtime OS checks.
• Failures in vswhere execution must be swallowed (log at most once at verbose or standard output) and not cause test failures.
• Provide disposal pattern: AssemblyResolve handler must be removable when the IDisposable returned by addVSAssemblyResolver is disposed (restore AppDomain handler list appropriately).

Acceptance Criteria:

• Running test suite without any VS* environment variables or VS installation does not hard fail at test discovery/startup.
• Duplicated code removed from the three target files and replaced with centralized helper usage.
• Setting FSHARP_VS_INSTALL_DIR to a mock root (parent of Common7/IDE) allows probing path construction (Common7/IDE/PublicAssemblies etc.).
• When multiple VS*COMNTOOLS environment variables exist, highest numeric version selected.
• vswhere-based detection works if no explicit environment variable present and vswhere.exe is available in standard installation path or ProgramFiles(x86)/Microsoft Visual Studio/Installer.
• Non-Windows platforms: HasVisualStudio=false; no exceptions.
• No changes to non-test directories.

Reference: Previous closed PR for similar broader changes: #18900

Please generate the necessary code changes and open a new pull request against main implementing the above test-only refactor.

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