Fix extraction of nested named expressions

[yangfan-yf-yf]

Description

Extracting i := a == (c := 5) can generate if i, c := new_func(a):, which is invalid Python. I fixed the named-expression visitor to traverse the assigned value so the existing extraction check detects the nested assignment and raises RefactoringError.

I updated the regression test that previously expected the invalid output. Single named-expression extraction and extraction of complete statements remain supported by the existing tests.

Fixes #871

Validation

• Windows, Python 3.12.3: python -m pytest -q — 2116 passed, 12 skipped, 5 xfailed.
• Extraction and use-function tests: 180 passed, 1 skipped, 1 xfailed.
• The corrected regression fails on the unpatched code (RefactoringError not raised).
• All five configured pre-commit checks passed for the changed files.

The first full run had two autoimport failures because my temporary directory was under an underscore-prefixed path. I reproduced both on unmodified master and reran the full suite successfully with a different temporary directory.

Checklist

• I have updated the regression test for the reported behavior.
• I have updated CHANGELOG.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.34%. Comparing base (2bd17a8) to head (c2047fb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #876   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files         134      134           
  Lines       26819    26819           
=======================================
  Hits        25570    25570           
  Misses       1249     1249           

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