Error when `--script` is passing a non-PEP 723 script (#11118)
## Summary
We now show a custom error if (1) the file doesn't exist at all, or (2)
it's not a PEP 723 script.
In the future, `uv lock --script` should probably initialize the script,
but that requires a more extensive refactor. At present, we just
silently lock the project instead, which is pretty bad!
Closes https://github.com/astral-sh/uv/issues/10979.
