.NET: fix(dotnet): validate declarative control-flow inputs - #9357

Motivation & Context

Fix inconsistent input validation in declarative workflow control-flow actions. Decision settings should follow the same value-validation contract as other action inputs, while preserving ordinary expressions and existing defaults.

Description & Review Guide

• What are the major changes? Apply existing validation helpers to condition and loop inputs, question settings, tool approval and automatic sending settings, agent external-loop conditions, HTTP timeouts, message selection, and variable-scope selection. Validate tool automatic-send settings before dispatch and preserve each invocation's evaluated setting across responses, approval, and checkpoint restoration, including replacement MCP approvals. Evaluate function automatic-send settings only for result processing, so restored approval denials complete without evaluating an unrelated setting. Register function automatic-send checkpoint state in the source-generated JSON context and cover real JSON save/resume with fresh workflows. Validate restored loop inputs, update the checked-in condition and all three foreach workflow fixtures, compile those loop fixtures for behavioral regression tests, and clarify the expression-evaluation contract.
• What is the impact of these changes? Invalid decision inputs raise DeclarativeActionException instead of being consumed without validation; invalid automatic-send settings do not dispatch tool requests. Pending invocations retain their original automatic-send behavior even when workflow state changes, including JSON checkpoints with reflection-based serialization disabled. Older approval denials clear pending state and record the denial; approved older requests still validate settings before invoking a function. Non-sensitive expressions, literal defaults, and metadata-preserving assignments keep their existing behavior. Add focused executor, checkpoint, generated-loop, and end-to-end regression tests, and keep configuration-loading tests focused on loading and assignment.
• What do you want reviewers to focus on? Consistency with the existing input-validation helpers, per-request response correlation, backward-compatible and source-generated JSON checkpoint restoration, default-value behavior, generated-loop parity, and ensuring invalid conditions do not execute either branch.

Validation from dotnet:

• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --filter-method '*RejectsSensitive*': failed as expected before the implementation fix (23 regression cases).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --filter-method '*ClearVariablesRejectsSensitiveScopeAsync': failed as expected before the corresponding implementation fix (1 regression case).
• dotnet build src\Microsoft.Agents.AI.Workflows.Declarative\Microsoft.Agents.AI.Workflows.Declarative.csproj --no-restore --tl:off: passed for all five target frameworks, with zero warnings and errors.
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --filter-class '*InvokeMcpToolExecutorTest': failed before the reapproval repair (6 regression cases); passed afterward (88 tests).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --filter-class '*ForeachExecutorTest': failed before the remaining fixture repairs (2 restore regression cases); passed afterward (26 tests).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore: failed before the initial review repairs (16 behavior regressions and 1 test-fixture failure); passed after all repairs and formatting (1,088 tests).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore -p:JsonSerializerIsReflectionEnabledByDefault=false --filter-method '*AutoSendSnapshotSurvivesJsonCheckpointAsync' --no-progress: failed before the JSON registration repair (4 cases with missing dictionary type information); passed afterward (4 tests).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --filter-method '*LegacyApprovalResponseValidatesAutoSendOnlyWhenApprovedAsync' --no-progress: failed before the denial repair (2 denial cases); passed afterward (4 denial/approved validation cases).
• dotnet test --project tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests -f net10.0 --no-restore --no-progress: passed after all repairs and formatting (1,096 tests).
• dotnet format src\Microsoft.Agents.AI.Workflows.Declarative\Microsoft.Agents.AI.Workflows.Declarative.csproj --no-restore: passed.
• dotnet format tests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests\Microsoft.Agents.AI.Workflows.Declarative.UnitTests.csproj --no-restore: passed.

Related Issue

N/A - maintainer-requested bugfix tracked internally; no public issue was opened. No competing open PR for this fix was found.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: GitHub Copilot assisted with implementation, regression tests, documentation, and this PR description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.