Fixes #14445.

Assertion rewriting keeps variables_overwrite state for walrus expressions so later expressions inside the same rewritten assertion can preserve evaluation order when a target is temporarily renamed. That state currently lives for the whole function scope, so a later assert in the same test can reuse an earlier walrus expression and evaluate it again.

This resets the overwrite state for each assertion. For the cases where the rewriter temporarily renames a walrus target to avoid clobbering a value too early, the generated code now restores the real target after the rewritten comparison/boolean operand has been evaluated. That preserves the walrus side effect for code after the assertion without leaking the expression into later assertions.

Tests run with PYTHONPATH=C:\hades\oss\pytest-14445\src so the local worktree was used:

• python -m pytest testing\test_assertrewrite.py::TestAssertionRewriteWalrusOperator::test_assertion_walrus_operator_value_changes_cleared_after_each_assert -q
• python -m pytest testing\test_assertrewrite.py -k walrus -q
• python -m pytest testing\test_assertrewrite.py::TestIssue11028 testing\test_assertrewrite.py::TestIssue11239 -q
• python -m pytest testing\test_assertrewrite.py -q
• python -m pytest testing\test_assertion.py -q
• external reproducer from Walrus expression duplicate evaluation failures with rewrite #14445 with the two walrus tests: passed
• python -m ruff check src\_pytest\assertion\rewrite.py testing\test_assertrewrite.py
• python -m ruff format --check src\_pytest\assertion\rewrite.py testing\test_assertrewrite.py
• git diff --check