Reject `pyproject.toml` in `--config-file` (#5842)
This already rejects `pyproject.toml`... but because the schema
validation is relaxed (we allow unknown fields, and all fields are
optional), a `pyproject.toml` doesn't get properly rejected here.
This PR makes the schema stricter, but in a safe way (by adding the
other `tool.uv` fields, like `workspace`, as any).
Closes #5832.
