Add unit tests for POST /index/message (Close #148)

[ciscokwiz]

Summary

Adds 7 pytest unit tests for the POST /index/message endpoint in apps/ai_agent/main.py, covering all code paths including error handling and resource cleanup.

Closes #148

Tests

#	Test	What it covers
1	test_weaviate_connection_failure_returns_503	Connection exception → 503 response
2	test_creates_collection_if_missing	Message collection auto-created when absent
3	test_inserts_new_message	collection.data.insert called, replace not called
4	test_replaces_existing_message	collection.data.replace called, insert not called
5	test_closes_weaviate_on_success	client.close() called after successful request
6	test_closes_weaviate_on_error	client.close() called in finally block on exception
7	test_missing_fields_returns_422	Pydantic validation returns 422 for empty body

Verification

All 7 tests pass:

tests/test_index.py::TestIndexMessage::test_weaviate_connection_failure_returns_503 PASSED
tests/test_index.py::TestIndexMessage::test_creates_collection_if_missing PASSED
tests/test_index.py::TestIndexMessage::test_inserts_new_message PASSED
tests/test_index.py::TestIndexMessage::test_replaces_existing_message PASSED
tests/test_index.py::TestIndexMessage::test_closes_weaviate_on_success PASSED
tests/test_index.py::TestIndexMessage::test_closes_weaviate_on_error PASSED
tests/test_index.py::TestIndexMessage::test_missing_fields_returns_422 PASSED

Notes

• No new runtime dependencies added; pytest and httpx are required only as dev dependencies.
• All Weaviate and OpenAI calls are mocked — no external services needed.
• Each write branch (insert vs replace) tested independently with assert_called_once / assert_not_called.

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