Add positive cDAC loading policy tests with .NET 11 dumps #5932

Background

Dbgshim now has deterministic negative/fallback coverage for the cDAC loading policies by pointing DOTNET_CDAC_PATH at a nonexistent library. The tests verify LegacyDacOnly, PreferCDac, and CDacOnly for both direct IXCLRDataProcess activation and the DBI ICorDebugProcess path.

The current DbgShim.UnitTests dump assets are .NET 6, so they cannot positively validate activation through mscordaccore_universal. Positive coverage requires dumps generated from a compatible .NET 11 runtime and the matching cDAC/DBI artifacts.

Proposed work

• Generate and publish Windows and Linux .NET 11 dump test assets using the existing diagnostics test-asset workflow.
• Make the matching mscordaccore_universal and mscordbi available to DbgShim.UnitTests.
• Add positive policy tests for both activation routes:
  • Direct data access requesting IXCLRDataProcess.
  • DBI activation requesting ICorDebugProcess.
• Verify policy behavior:
  • CDacOnly succeeds without invoking the library provider.
  • PreferCDac succeeds through cDAC without invoking the fallback provider.
  • LegacyDacOnly uses the legacy DAC provider and succeeds.
• Include Windows-hosted Linux cross-dump coverage so target platform detection and cDAC activation are exercised together.
• Assert which path was used, not only that activation succeeded, using provider invocation counts and/or a test-visible activation signal.

Acceptance criteria

• Positive cDAC activation is exercised in CI against a .NET 11 dump.
• Direct data-access and DBI routes are both covered.
• All three loading policies have independently identifiable test cases.
• The tests prove cDAC success does not consult the fallback provider.
• Legacy DAC behavior remains covered and unchanged.