fix: reject invalid chat summary unread counts

[shuxueshuxue]

Summary

• fail loudly when ChatToolService list_chats receives a malformed unread_count projection
• stop before unread filtering/rendering can leak raw comparison errors

Verification

• uv run python -m pytest tests/Integration/test_messaging_social_handle_contract.py -q -k integer_unread_count
• 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
• 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