feat(core): add agent-to-agent communication

[Subhajitdas99]

Summary

Adds authenticated agent-to-agent communication within a tenant.

Changes

• Add POST /v1/a2a/messages
• Require agent-scoped API keys for A2A communication
• Derive the sender exclusively from the authenticated API key
• Prevent cross-tenant recipient access
• Prevent agents from messaging themselves
• Persist A2A messages through the existing event write path
• Preserve session and correlation identifiers
• Support optional message metadata
• Add comprehensive A2A tests

Testing

• python -m pytest core/tests/test_a2a.py -v — 9 passed
• python -m pytest core/tests/test_a2a.py core/tests/test_route_authz.py core/tests/test_tenant_isolation.py -v — 24 passed
• python -m compileall core — passed
• git diff --check — passed

[Subhajitdas99]

Thanks! All three CI checks are now passing and the branch is conflict-free. The A2A implementation is ready for review.

[saadamjad]

@Subhajitdas99 could you please confirm how this is currently working? It would be great if you could also share some screenshots of the working flow.

[Subhajitdas99]

Hi @saadamjad, sure. The current A2A flow works through POST /v1/a2a/messages.

The request is authenticated using an agent-scoped API key.
The sender agent is derived from the authenticated API key rather than being accepted from the request body.
The recipient is looked up using both tenant_id and agent_id, so cross-tenant recipients are rejected.
An agent cannot send a message to itself.
Once validated, the message is persisted through the existing write_event() path as an a2a.message event.
session_id, correlation_id, and optional metadata are preserved.
The existing event pipeline can then handle embedding/indexing as it normally does.

I tested the implementation with 9 dedicated A2A tests and the related authorization/tenant-isolation tests (24 total), and all three CI checks are passing.

I'll add screenshots showing the successful request and the authorization/tenant-isolation behavior.