Fix extraction of nested named expressions

[yangfan-yf-yf]

Description

Extracting an expression with nested named expressions can produce invalid code such as if i, c := new_func(a):. The named-expression visitor counted the outer assignment without visiting its value, so it missed the inner assignment.

I updated the visitor to inspect the value and reject this unsafe extraction with RefactoringError. The regression verifies the refusal for the reported nested expression. The existing single-assignment extraction behavior remains covered by the suite.

Fixes #871

Checklist

• I have added tests that prove my fix is effective
• I have updated CHANGELOG.md

Validation

Synced with current upstream master 22f5e501, including the updated patchedast implementation and contribution guidelines.

• Windows, Python 3.12.3: extraction, usefunction, and patchedast tests — 345 passed, 11 skipped, 1 xfailed.
• Full suite — 2,149 passed, 12 skipped, 5 xfailed.
• All five pre-commit checks and diff checks pass.
• An actual execution probe confirms that the unsafe extraction raises RefactoringError, leaves the source unchanged, and preserves the original program's output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.34%. Comparing base (22f5e50) to head (9b427bf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #876   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files         134      134           
  Lines       26762    26762           
=======================================
  Hits        25516    25516           
  Misses       1246     1246           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.