[Feat]: Add protocol conformance fixtures for cross-implementation interoperability

### Context
A2A interoperability will benefit from shared, versioned conformance fixtures that different implementations can run in CI.

### Proposal
Introduce a small `conformance-fixtures/` pack (or companion repo) with protocol-level test vectors.

Starter fixture categories:
1. Agent Card validation examples (valid/invalid)
2. Capability negotiation edge cases
3. Message routing and correlation IDs
4. Error payload normalization
5. Security-related negative cases (malformed claims, unsupported transport assumptions)

### Expected outcome
- Faster implementation consistency across languages
- Easier compatibility regression checks
- Clearer path for external contributors to add new edge-case vectors

If useful, I can draft an initial fixture structure and contribution pattern.