Add conformance fixtures for canonical A2C messages and validation failures

[king-tero]

Summary

The repo currently has a useful prose specification and a Python implementation, but no shared conformance fixtures for third-party implementers.

Suggested coverage

A small conformance/ or testdata/ directory could include JSON fixtures for:

• valid HANDSHAKE, HANDSHAKE_ACK, PROMPT, ACTION, PENDING, TOOL_RESULT, RESPONSE, ERROR, GOODBYE, and INTERRUPT messages;
• invalid messages with missing universal fields;
• invalid seq regressions/gaps;
• undeclared tool calls;
• denied, timeout, cancelled, and failed tool results;
• streaming RESPONSE chunks with final sentinel;
• Daemon Mode handshakes.

Why it matters

A2C is intended as an interoperability protocol. Conformance fixtures would help other implementations validate behavior without reverse-engineering the Python reference implementation.

Possible fixes

• Add canonical JSON fixtures under conformance/messages/.
• Add a minimal Python test suite that validates fixtures against the Pydantic models.
• Document which fixtures are normative examples and which are negative tests.