test: Cover Python quality task graph (#13675)
### Description
Part 12 of 13 in stack #13677, based on #13674. Adds fixtures and
end-to-end graph coverage for workspace fan-out, qualified filters,
formatter warnings, heterogeneous members, all-package entrypoints,
aggregate argument rejection, query visibility, and unchanged lockfiles.
Previous: #13674. Next: #13676.
### Testing Instructions
- `cargo test -p turbo --test uv_workspace_test --no-fail-fast` passed:
13 tests.
- Full-tip formatting, 437 repository tests, 135 engine tests, 12
entrypoint tests, and strict clippy also passed.
- Every cumulative tip passed `cargo check --tests`.
The declared-tool fixture cases are dry-run graph tests. `uv` is
unavailable, so real declared-tool execution is not covered, and a full
workspace run was not performed.
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
