Search beyond workspace root when discovering configuration (#5931)
## Summary
Previously, we wouldn't respect configuration files in directories
_above_ a workspace root. But this is somewhat problematic, because any
`pyproject.toml` will define a workspace root...
Instead, I think we should _start_ the search at the workspace root, but
go above it if necessary.
Closes: #5929.
See: https://github.com/astral-sh/uv/pull/4295.
