Add `uv publish --dry-run` (#15638)
## Summary
`uv publish --dry-run` will perform the `--check-url` validation, and
hit the `/validate` endpoint if the registry is known to support
fast-path validation (like pyx). The `/validate` endpoint lets us
validate an upload without uploading the file _contents_, which lets you
skip the expensive step for common mistakes.
In the future, my hope is that the `/validate` step will deprecated in
favor of Upload API 2.0.
