Fix PerformanceCounterLib cache races

[jaideeppyne]

Fixes #90803.

Root cause

CloseAllTables can clear a cache field after its getter has initialized it but before the getter rereads the field for its return value. That turns a successful initialization into a null return. Separately, CloseAllTables enumerates s_libraryTable without the lock used by GetPerformanceCounterLib, so concurrent library creation can mutate the Hashtable during enumeration.

Changes

• Capture each initialized category, name, and help table in a local and return that stable snapshot.
• Take InternalSyncObject while enumerating and clearing the shared library table.

No public API or intended cache-invalidation behavior changes. A caller already using a cache may finish using that snapshot while invalidation causes subsequent callers to rebuild it.

Validation

• git diff --check — passed.
• ./dotnet.sh build src/libraries/System.Diagnostics.PerformanceCounter/src/System.Diagnostics.PerformanceCounter.csproj -f net11.0-windows -c Release --no-restore — passed with 0 warnings and 0 errors.

The runtime behavior and test suite are Windows-only, so they cannot execute on the macOS development host; the Windows-target implementation and its analyzers were compiled successfully. CI provides the Windows runtime coverage.

Risk and rollback

Risk is limited to synchronization in the internal performance-counter cache. The additional lock reuses the lock already taken for every s_libraryTable mutation. Rollback is a single commit revert.

AI assistance was used for repository exploration and drafting; I reviewed the patch and validation results and will personally address review feedback.

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