fix: reject empty created direct chat ids

[shuxueshuxue]

Summary

• make ChatToolService direct send reject created chat ids that are empty or non-string
• assert unread checks and send side effects do not run when direct-chat creation returns an invalid id
• record the checkpoint in mycel-db-design commit a33db52

Verification

• RED: uv run python -m pytest tests/Integration/test_messaging_social_handle_contract.py -q -k created_chat_id_is_empty -> failed with DID NOT RAISE before implementation
• GREEN: uv run python -m pytest tests/Integration/test_messaging_social_handle_contract.py -q -k created_chat_id_is_empty -> 1 passed
• uv run python -m pytest tests/Integration/test_messaging_social_handle_contract.py tests/Unit/backend/web/services/test_chat_delivery_hook.py tests/Unit/messaging/test_chat_delivery_dispatcher.py -q -> 92 passed
• uv run ruff format messaging/tools/chat_tool_service.py tests/Integration/test_messaging_social_handle_contract.py --check
• uv run ruff check messaging/tools/chat_tool_service.py tests/Integration/test_messaging_social_handle_contract.py
• .venv/bin/python -m pyright messaging/tools/chat_tool_service.py
• git diff --check