[refurb] Suppress FURB122 fix when walrus operator rebinds a loop variable

[kpeis695]

Summary

Fixes #21107.

When write_arg contains a named expression (walrus operator :=) whose target is one of the for-loop iteration variables, the FURB122 fix converts:

for line in src:
    dst.write(line := line.upper())

to:

dst.writelines(line := line.upper() for line in src)

which is a SyntaxError — PEP 572 forbids an assignment expression from rebinding a comprehension iteration variable.

This PR adds a write_arg_rebinds_loop_var helper that walks write_arg with any_over_expr, looking for an ExprNamed whose target name matches any loop iteration variable. When found, the diagnostic is skipped entirely to avoid emitting a broken fix.

The same guard covers tuple-destructured loop targets (e.g. for first, *rest in src: dst.write(rest := ...)).

Walrus operators that bind a different name (not a loop variable) still produce the diagnostic and fix, as before.

Test Plan

Added four test cases to FURB122.py:

• Three "OK" cases: simple loop var rebind, tuple loop var rebind, one-of-two loop vars rebind — all suppressed.
• One "Error" case: walrus binds other (not a loop var) — fix still applies.

cargo test -p ruff_linter -- refurb passes (44/44).

[kpeis695]

Same issue as the PYI036 PR. Branched incorrectly and picked up unrelated commits. Reopened from a clean branch with only the FURB122 fix.

[ntBre]

Thanks for reopening these, but I'm going to convert them back to drafts for now. For a first-time contribution, let's focus on getting one PR to a resolution before opening more for review. Let's start with #26354 since I've already started reviewing it.