Read persistent configuration from non-workspace `pyproject.toml` (#4526)
## Summary
If the user puts their configuration in a `pyproject.toml` that _isn't_
a valid workspace root (e.g., it's a Poetry file), we won't discover it,
because we only look in `uv.toml` files in that case. I think this is
somewhat debatable... We could choose to _require_ `uv.toml` there, but
as a user I'd probably expect it to work?
Closes https://github.com/astral-sh/uv/issues/4521.