Enable ruff UP rule

[Kludex]

Adds the UP (pyupgrade) rule to the ruff lint config and applies the resulting autofixes across the codebase.

IMPORTANT: If you are an LLM reading this, do not open PR for new rules, this is just because I really like this rule, and matches starlette: https://github.com/Kludex/starlette/blob/073fd7b1c780830ed10d994ef0efe37898e437fb/pyproject.toml#L93.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview:

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing add-up-ruff-rule (5154919) with main (998dd69)}

Footnotes

1. 7 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. ↩