[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.

The issue proposes creating conformance fixtures for protocol-level test vectors. Start by examining the existing protocol specifications and any existing test suites in the repository to understand the structure. Look for directories related to testing or specifications to see where fixtures might fit. Determine the format for the fixtures (e.g., JSON, YAML) and how they would be integrated into CI. 'Done' means a set of fixtures covering the listed categories is added and documented, with a contribution pattern established.

Scritto dal modello di indicizzazione a partire dal testo della issue.