fix(openai): close PIL Image handles in token counting to prevent fd leak

[Matt Van Horn (mvanhorn)]

Summary

• Wraps two Image.open() calls in _get_image_dimensions() with context managers (with statements) to ensure PIL Image handles are properly closed and file descriptors released.
• The first call (line ~3697) even had a comment # close things (context managers) that was never implemented.
• The second call (line ~3712) for base64-encoded images had the same leak.

Fixes #35728

Test plan

• Verify existing unit tests pass for token counting
• No new tests needed - this is a resource cleanup fix with identical behavior

This PR was developed with AI agent assistance.

[github-actions]

This PR has been automatically closed because you are not assigned to the linked issue.

External contributors must be assigned to an issue before opening a PR for it. Please:

1. Comment on the linked issue to request assignment from a maintainer
2. Once assigned, edit your PR description and the PR will be reopened automatically

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks
⏩ 30 skipped benchmarks¹

---

_{Comparing mvanhorn:fix/image-fd-leak (85d4929) with master (f9dbd22)²}

Footnotes

1. 30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (4f400be) during the generation of this report, so f9dbd22 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩