Enable ruff I (import sorting) rule

[bdraco]

What do these changes do?

Enables the I (import sorting) rule in [tool.ruff.lint]
alongside the existing UP (pyupgrade) group, and updates the
ruff-check pre-commit hook to run with
--fix --exit-non-zero-on-fix --show-fixes so import-order fixes
apply on commit (and abort the commit when something changed). This
follows the same pattern as yarl PR
#1698: one tool, one
rev, for both formatting and import sorting.

Sorted imports in six files where ruff check --select I --fix
found drift (tests/conftest.py,
tests/isolated/multidict_pop.py,
tests/test_circular_imports.py,
tests/test_mutable_multidict.py,
benchmarks/benchmark.py, benchmarks/istr.py).

AGENTS.md updated to reflect the additional lint group, so the
"only the UP group enabled" guidance no longer contradicts the
config.

Are there changes in behavior for the user?

No, contributor tooling only.

Is it a substantial burden for the maintainers to support this?

No. One additional lint rule sharing the existing [tool.ruff]
config; no new pre-commit hooks were added.

Related issue number

N/A; mirrors the approach taken in yarl #1698.

Checklist

• I think the code is well written
• Unit tests for the changes exist: N/A (tooling-only change; ran the touched test modules and the full pre-commit suite to verify)
• Documentation reflects the changes: N/A (no user-visible API change; AGENTS.md updated)
• If you provide code modification, please add yourself to CONTRIBUTORS.txt: N/A (no such file in this repo)
• Add a new news fragment into the CHANGES/ folder (CHANGES/1348.contrib.rst)

Agent run details (optional, for reviewers)

Drafted with Claude Code (Opus 4.7); reviewed by @bdraco.

Verified:

$ ruff check multidict/ tests/ benchmarks/
All checks passed!
$ ruff format --check multidict/ tests/
29 files already formatted
$ python3 -m pytest tests/test_circular_imports.py tests/test_mutable_multidict.py --no-cov -q -W ignore::UserWarning
137 passed in 0.30s
$ pre-commit run --all-files
all hooks passed (including mypy py311 + py313)
$ make doc-spelling
5 pre-existing CHANGES.rst misspellings only; CHANGES/1348.contrib.rst is clean

[codspeed-hq]

Merging this PR will not alter performance

✅ 244 untouched benchmarks

---

_{Comparing bdraco:ruff-add-isort-rule (dfff2af) with master (fc6933d)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (fc6933d) to head (dfff2af).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1348   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          28       28           
  Lines        3627     3627           
  Branches      265      265           
=======================================
  Hits         3622     3622           
  Misses          3        3           
  Partials        2        2           

Flag	Coverage Δ
CI-GHA	99.86% <100.00%> (ø)
pytest	99.86% <100.00%> (ø)

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