[flake8-comprehensions] NFKC-normalize keyword names in C408 fix

[baltasarblanco]

Python normalizes identifiers to NFKC, but does not normalize string literals. The fix for unnecessary-collection-call (C408) reparses the call with libcst, which — unlike Ruff's own parser — does not normalize identifiers, and then emits the raw source text of each keyword argument as a dictionary key. This changed the key at runtime: dict(ℼ=3.14) has the key π, but was rewritten to {"ℼ": 3.14}.

Normalize the keyword name before quoting it, matching what fix::codemods already does for qualified names built from libCST nodes.

Fixes #16234

Summary

Python normalizes identifiers to NFKC, but not string literals. The C408 fix
re-parses the call with libcst, and that parser has a particular quirk: it does
not normalize (Ruff's own parser does). So libcst hands over the raw source text
of the kwarg as the key.

dict(ℼ=3.14) has the key π at RUNTIME, but was rewritten to {"ℼ": 3.14} >
this changes the behavior.

My fix normalizes the kwarg name before quoting it. I'm not creating anything
new, because fix::codemods already does the same thing.

I decided not to mark it as unsafe, because the C408 fix is already unsafe. And
unlike B009/B010/B043 (string > identifier, where preserving the behavior is
impossible), C408 goes identifier > string. So the correct key can be computed :)

Test Plan

• 4 new cases at the end of C408.py: ℼ→π, ſ→s, 𝕒→a, plus an ASCII control with no changes.
• The snapshot diff is essentially additive. No diagnostic was altered.
• The allow_dict_calls_with_keyword_arguments snapshot stayed INTACT.
• cargo test -p ruff_linter: 2811 passed, 0 failed :)
• clippy --workspace --all-features -D warnings: 100% clean. generate-all: up-to-date. And prek run -a: 14/14.

Fixes #16234

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[ntBre]

Thanks! One very small nit, which I'll just go ahead and apply, assuming it doesn't break anything!