Tolerate missing `[project]` table in `uv venv` (#6178)
## Summary
Fixes #6177
This ensures a `pyproject.toml` file without a `[project]` table is not
a fatal error for `uv venv`, which is just trying to discover/respect
the project's `python-requires` (#5592).
Similarly, any caught `WorkspaceError` is now also non-fatal and instead
prints a warning message (feeback welcome here, felt less surprising
than e.g. a malformed `pyproject.toml` breaking `uv venv`).
## Test Plan
I added two test cases: `cargo test -p uv --test venv`
Also, existing venv tests were failing for me since I use fish and the
printed activation script was `source .venv/bin/activate.fish` (to
repro, just run the tests with `SHELL=fish`). So added an insta filter
to normalize that.