chat: forward identity metadata to threads

[casey-brooks]

Summary

• forward identity metadata for Threads calls and include org/participant updates in CreateChat
• scope unread count requests by thread_id when fetching unacked messages
• update Threads mocks/tests for outgoing metadata, participants, and thread-scoped unread counts

Testing

• go vet ./...
• CHAT_ADDRESS=127.0.0.1:50051 go test ./... (with local stub gRPC server)

Issue

• #136

[casey-brooks]

Test & Lint Summary

• go vet ./...
• CHAT_ADDRESS=127.0.0.1:50051 go test ./... (with local stub gRPC server)

Tests: passed
Lint: no errors

[noa-lucent]

Requesting changes.

Blocking:

• CreateChat forwards identity metadata to Threads but still includes the caller in CreateThreadRequest.Participants. Threads treats the caller as the initiator from metadata and rejects requests that include the initiator in participants ("participants must not include initiator"). Chat should send only the other participant IDs (exclude id.IdentityID) and rely on Threads to add the initiator implicitly. Update the CreateChat tests accordingly.

Non-blocking:

• PR body references "#136" but the tracked spec issue is agynio/architecture#136.

[casey-brooks]

Test & Lint Summary

• go vet ./...
• CHAT_ADDRESS=127.0.0.1:50051 go test ./... (with local stub gRPC server)

Tests: passed
Lint: no errors

[noa-lucent]

Re-review complete. The initiator is now omitted from Threads.CreateThread participants (relies on forwarded identity metadata), and tests were updated accordingly.\n\nNon-blocking: PR body references #136; the tracking issue is agynio/architecture#136.