.NET: Simplify ClientHeadersScope to rely on AsyncLocal natural restoration

[rogerbarreto]

Summary

Simplifies ClientHeadersScope by dropping the explicit using/Dispose pattern and relying on the natural AsyncLocal<T> restoration that happens when an awaited async method returns.

What changes

• ClientHeadersScope collapses to a single Current { get; set; } property over an AsyncLocal<IReadOnlyDictionary<string, string>?>. Drops Push, the Scope struct, and Dispose. XML doc explains the AsyncLocal natural-restoration semantics so the design intent is self-documenting.
• ClientHeadersAgent uses a direct ClientHeadersScope.Current = snapshot before delegating in both RunCoreAsync and RunCoreStreamingAsync. Drops the local RunAsyncCoreAsync helper and the snapshot-passed-as-parameter dance.
• Test 10 renamed to ClientHeadersScope_IsAsyncLocalIsolatedAndAutoRestoresAsync; keeps the parallel-isolation assertion and adds a "caller sees null after the awaited probe returns" assertion.
• Test 12 switches from using ClientHeadersScope.Push(...) to direct Current = ... with try/finally for test isolation.

Why

AsyncLocal<T> mutations made inside an awaited async method are visible within that method (and all of its awaits) but do not propagate back to the caller after the method returns. Since ClientHeadersAgent is the only caller of the scope and both call sites are async methods awaited by their callers, the explicit using/Dispose pattern was doing work the runtime already does for us.

The snapshot deep-copy in TrySnapshot stays. It defends against caller mutating the source Dictionary mid-run, which is independent of the AsyncLocal restoration mechanism.

Verification

• Foundry build clean (0 errors, 0 warnings).
• 401 Foundry unit tests pass.
• CI-parity dotnet format --verify-no-changes clean on the changed project.

[github-actions]

Automated Code Review

Reviewers: 2 | Confidence: 95% | Result: All clear

Reviewed: Test Coverage, Design Approach

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