Annotate all DllImport/LibraryImport/UnmanagedCallersOnly with [SupportedOSPlatform] #132776

Summary

Annotate platform-specific DllImport / LibraryImport / UnmanagedCallersOnly declarations with [SupportedOSPlatform] / [UnsupportedOSPlatform].

Motivation

The wasm CoreCLR ManagedToNativeGenerator (src/tasks/WasmAppBuilder/coreclr/PInvokeCollector.cs) already skips P/Invokes and callbacks that are unsupported on the target OS, evaluating SupportedOSPlatform / UnsupportedOSPlatform at method → declaring-type → assembly scope against the target (browser / wasi).

However, most low-level Interop.* declarations are not annotated — e.g. Interop.Kernel32.GetCurrentProcess lives in the cross-platform System.Private.CoreLib with no platform attribute. As a result they flow into the scan and produce WASM0066 noise on browser/wasi (kernel32, ntdll, advapi32, …), and more generally they are invisible to platform-compatibility tooling.

Annotating these declarations would let the existing wasm platform-skip eliminate the noise at the source, and would improve CA1416 platform-compatibility analysis across the board.

The hard part — callers

Annotating the interop leaves is not enough; the callers must be solved too.

Adding [SupportedOSPlatform("windows")] (etc.) to the low-level interop will surface CA1416 on every caller. Many callers are already guarded (if (OperatingSystem.IsWindows())) and will be fine, but some are not, and cross-cutting annotation requires walking and annotating/guarding those call chains. This is the bulk of the work and must be scoped in — it cannot be done by annotating leaves alone.

Scope

• src/libraries/Common/src/Interop/** and the per-assembly interop partials.
• Coordinate with the platform-compatibility analyzer (CA1416) expectations.
• Enables (and is complemented by) the wasm relink-gating work in the companion issue.