PIX validation fallback should inspect canonical DXIL

The PIX test validation helper directly validates the canonical DFCC_DXIL container part, but its metadata-stripping fallback parses DFCC_ShaderDebugInfoDXIL (ILDB). Those parts can diverge in content that survives debug-info stripping, so the fallback can classify validation using a different module from the one the validator rejected.

Reproduction

1. Construct a DXIL container whose canonical DFCC_DXIL and DFCC_ShaderDebugInfoDXIL parts contain different unused metadata.
2. Put a real non-PIX validation defect in the canonical part while the debug-info part contains only metadata permitted by the PIX test helper.
3. Run the helper's direct validation and metadata-stripping fallback.

Direct validation rejects the canonical module, but the fallback inspects the ILDB module. In a divergent container, this can excuse or misclassify a genuine canonical validation failure.

Proposed fix

Parse, clone, strip, and revalidate the same canonical DFCC_DXIL part targeted by direct validation. Add a focused divergent-container regression test plus a positive control for canonical known-PIX metadata.

This is a test-harness correctness issue, not a compiler behavior change. It was discovered while reviewing PR #8841 and should be handled in a separate follow-up commit rather than expanding that review-feedback amendment.

Start at the PIX test validation helper's direct validation and metadata-stripping fallback, comparing how each selects the DFCC_DXIL and DFCC_ShaderDebugInfoDXIL parts. Add the divergent-container regression test and canonical known-PIX positive control described in the issue; done means the fallback parses, clones, strips, and revalidates the canonical DFCC_DXIL module.