Add test coverage for walrus with allow-redefinition

[popsiclelmlm]

Summary

• Add regression coverage for assignment expressions used with --allow-redefinition.
• Cover variables introduced by walrus expressions and then redefined to a different type.

Reproduction

• Issue PEP 572: Test interaction with --allow-redefinition #7314 tracks the missing PEP 572 interaction coverage for --allow-redefinition.
• The added cases exercise if y := x: and if z := 1: followed by cross-type redefinition.

Root cause

• Existing tests covered assignment-expression branch merging, but did not cover subsequent redefinition of walrus-introduced variables under --allow-redefinition.

Changes

• Extended testNewRedefineAssignmentExpression in test-data/unit/check-redefine2.test with two focused cases.

Tests

• .venv/bin/python -m pytest -n0 -q mypy/test/testcheck.py::TypeCheckSuite::check-redefine2.test -k NewRedefineAssignmentExpression
• .venv/bin/python -m pytest -n0 -q mypy/test/testcheck.py::TypeCheckSuite::check-redefine2.test
• .venv/bin/pre-commit run --files test-data/unit/check-redefine2.test
• git diff --check

Screenshots/Logs

• Not applicable; test-only change.

Disclosure

• I used an LLM assistant while preparing this test-only change and reviewed the diff and test results before submission.