[codex] Add walrus allow-redefinition test

[EltonChang1]

Summary

Adds focused coverage for #7314 by testing how assignment expressions inside a list comprehension interact with --allow-redefinition.

The new case verifies that the comprehension result is inferred as list[str], that mypy currently keeps the outer variable's revealed type as int after the comprehension, and that a later assignment can still redefine it to float under --allow-redefinition.

Test Plan

• .venv/bin/python -m pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-redefine2.test -k testNewRedefineAssignmentExpressionComprehensions
• .venv/bin/python -m pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-redefine2.test

[Copilot]

Pull request overview

Adds a new unit test intended to cover the interaction between --allow-redefinition and assignment expressions used inside a list-comprehension-like construct, as discussed in #7314.

Changes:

• Adds a new check-redefine2 test case asserting that a walrus assignment can redefine a local variable’s type under --allow-redefinition.
• Asserts that a subsequent reassignment can redefine the same variable again to a different type.

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