pantsbuild / pants / 32191341339

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write sidecar lockfile metadata files with a trailing newline (#23631)

## Problem

Sidecar `*.lock.metadata` files (written when
`[python].separate_lockfile_metadata_file` is enabled) have no final
newline.

Both write sites encode `LockfileMetadata.to_json()` directly:

```python
FileContent(
    PythonLockfileMetadata.metadata_location_for_lockfile(req.lockfile_dest),
    metadata.to_json(with_description=descr).encode(),
)
```

and `to_json` is a bare `json.dumps(metadata_dict, ensure_ascii=True,
indent=2)`, which does not emit a trailing newline. The embedded-header
path never hit this because `add_header_to_lockfile` splices the
metadata into the lockfile with its own newlines.

The result is that `git diff` reports `\ No newline at end of file` on
every regenerated metadata file, and any repo-wide end-of-file fixer
(pre-commit's `end-of-file-fixer`, `editorconfig`, prettier) rewrites
them — so they churn back and forth between `generate-lockfiles` runs
and commits.

39 of the 53 `*.lock.metadata` files checked into this repo are
currently missing the newline.

## Fix

- Append the newline at both write sites (`generate_pex_lockfile` and
`generate_uv_lockfile`).
- Update the 39 bundled `*.lock.metadata` files so they match what
regeneration now produces.
- Assert the trailing newline in the existing sidecar tests.

The read path is unaffected — it goes through `json.loads`, which
tolerates trailing whitespace.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01MNsGW5gQyiVuFAD64LEWVs

---------

Co-authored-by: Jason Barnett <jason.barnett@altana.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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