fix: reject invalid chat unread counts

[shuxueshuxue]

Summary

• fail loudly when ChatToolService send_message receives a malformed count_unread result
• preserve the read-before-send gate by stopping before send side effects

Verification

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