[cDAC] Tolerate missing WKS card-table slot in reduced dumps

[steveisok]

Windows CDB /mw single-file dumps can omit the workstation GC card-table pointer slot while retaining the GC data needed for heap walking. Treat that slot as optional and report a null card table when unreadable, allowing IGC.GetHeapData and GetGCHeapStaticData to continue returning valid heap and generation data.

Also narrows debug cross-validation for this known mismatch and adds coverage for readable and missing card-table slots.

Tests: GCTests (36 passed)

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What changed in this PR

This PR updates the cDAC GC workstation heap contract to treat the GCHeapCardTable global slot as optional when the slot’s target memory can’t be read, returning a null card table pointer while still providing valid heap and generation data.

Changes:

• Make GCHeapWKS.CardTable resilient to an unreadable card-table slot by using TryReadPointer and falling back to TargetPointer.Null.
• Relax DEBUG cross-validation in SOSDacImpl.GetGCHeapStaticData to tolerate a zero card table when the legacy DAC still reports a non-zero value.
• Add unit test coverage for both readable and unreadable card-table slot scenarios, including validating GetGCHeapStaticData output.

File	Description
src/​native/​managed/​cdac/​tests/​UnitTests/​GCTests.cs	Adds tests for readable vs unreadable workstation card-table slot, and validates GetGCHeapStaticData returns a null card table without breaking generation data.
src/​native/​managed/​cdac/​Microsoft.Diagnostics.DataContractReader.Legacy/​SOSDacImpl.cs	Narrows DEBUG-only cross-validation to allow card_table == 0 for reduced-dump scenarios in GetGCHeapStaticData.
src/​native/​managed/​cdac/​Microsoft.Diagnostics.DataContractReader.Contracts/​Contracts/​GC/​GCHeapWKS.cs	Switches card-table read to TryReadPointer with a TargetPointer.Null fallback to tolerate unreadable target memory.

[rcj1]

I don't think this is the root cause of the issue; rather, we are trying to read memory at gc_heap::card_table which we do not preserve in a dump. g_card_table, however, is preserved but cDAC is not reading from it.