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.