Validator: migrate advisory baseline from file-based to annotation-based

[arjunmehta-git]

Context

The advisory baseline file (`build/validate-schemas.advisory-baseline.txt`) suppresses known issues by matching exact `file\tmessage` strings.

Problem

• Fragile: any message wording change invalidates the baseline entry
• Opaque: the exemption lives in a separate file, not with the schema
• Hard to audit: reviewers must cross-reference the baseline file to understand why a warning is suppressed

Proposal

Replace with per-schema/per-operation annotations:

```yaml

Exempt from Rule 28 (response code semantics) — intentional 200 for upsert

x-advisory-exempt: [28]
```

The validator would check for `x-advisory-exempt` on the operation or schema and skip the listed rule numbers.

Benefits:

• Self-documenting — the exemption lives with the schema
• Survives message rewording
• Easy to audit in code review

Impact

• Validator change: add `x-advisory-exempt` support, remove baseline file loading
• Schema changes: add annotations to constructs currently in the baseline file (currently 8 entries across 4 constructs)
• Delete `build/validate-schemas.advisory-baseline.txt`

References

Identified in schema validator audit — Section 3e (Advisory Baseline File).