Support PEP 723 metadata with `uv run -` (#8111)
## Summary
Fixes #8097. One challenge is that the `Pep723Script` is used for both
reading
and writing the metadata, so I wasn't sure about how to handle
`script.write`
when stdin (currently just ignoring it, but maybe we should raise an
error?).
## Test Plan
Added a test case copying the `test_stdin` with PEP 723 metadata.
