test: add unit tests for /chat endpoint

[lireno]

Summary

Adds comprehensive unit and integration tests for the /chat endpoint and its underlying functions (openai_chat_stream, convert_to_openai_messages), which previously had zero test coverage.

Changes

Unit tests for convert_to_openai_messages

• Converts simple user messages
• Converts assistant messages
• Converts multiple messages
• Converts messages with tool invocations (including function_call and function_call_output)
• Handles tool invocations without results
• Handles mixed messages (regular + tool invocations)
• Handles empty messages list

Unit tests for openai_chat_stream

• Streams text delta events as SSE-formatted strings
• Streams tool call events (output_item.added, function_call_arguments.delta, function_call_arguments.done)
• Handles edge case where only one event exists in the stream

Integration tests for /chat endpoint

• Happy path: validates streaming response with proper SSE format and x-vercel-ai-data-stream header
• Edge case: empty messages list
• Edge case: missing API key (403 Forbidden)
• Edge case: invalid API key (401 Unauthorized)
• Edge case: missing required fields (422 Unprocessable Entity)
• Edge case: messages containing tool invocations

Testing

The tests follow existing patterns in the codebase:

• Pytest fixtures from conftest.py for database and authentication
• unittest.mock.patch for mocking the OpenAI client in openai_chat_stream
• FastAPI TestClient for integration tests
• Same directory structure as other route tests

To run:

cd backend
docker compose exec test-runner pytest aci/server/tests/routes/agent/ -v

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