fix(ui): accept top-level query requests

[Gujiassh]

Summary

Fixes Web UI query handling for clients that send the query text at the top level of the request payload.

UserRequest previously accepted only the nested form:

{"type": "query", "content": {"query": "..."}}

Some clients, including demo/dummy_client.py, send:

{"type": "query", "query": "..."}

That shape parsed with content=None, which later caused AttributeError: 'NoneType' object has no attribute 'query' in the WebSocket query worker. This change normalizes that legacy/top-level query form into content during request validation while keeping the existing nested form unchanged.

Fixes #223.

Validation

• UV_PYTHON=/usr/bin/python3.12 uv run pytest tests/ui/test_user_request.py -q
• UV_PYTHON=/usr/bin/python3.12 uv run ruff check utu/ui/common.py tests/ui/test_user_request.py
• UV_PYTHON=/usr/bin/python3.12 uv run ruff format --check utu/ui/common.py tests/ui/test_user_request.py
• git diff --check

Additional note: UV_PYTHON=/usr/bin/python3.12 uv run pytest tests/ui/test_user_request.py tests/utils/test_print_utils.py -q passes the new UI request tests, but the existing tests/utils/test_print_utils.py cases fail in this non-interactive shell because prompt_toolkit raises EOFError when stdin is not a TTY.