Validator: fix false positives in duplicate schema fingerprinting (Rule 29)

[arjunmehta-git]

Context

Rule 29 detects duplicate schemas across constructs using a fingerprint based on property names and types.

Problem

Two schemas with the same property names but different:

• `$ref` targets
• Constraints (`minLength`, `pattern`, etc.)
• `required` arrays
• `description` values

...would produce the same fingerprint and be falsely flagged as duplicates.

Proposal

Improve the fingerprint function to include:

• The full `$ref` path (not just the schema name)
• The `required` array (sorted)
• Constraint presence (e.g., has `minLength` vs doesn't)

This reduces false positive matches while still catching genuinely duplicated schemas.

Impact

Validator-only change. May surface new true duplicates or suppress current false positives.

References

Identified in schema validator audit — Section 4 (Robustness Issues).