Fix walrus double eval

[shuckc]

Addresses issue #14445

Root cause: The assertion rewriter's visit_NamedExpr returned the ast.NamedExpr node directly, which was then referenced in multiple places in the rewritten AST (the comparison statement, the results tuple for _call_reprcompare, and format expressions). Each reference re-evaluated the walrus operator at runtime, causing side effects to fire multiple times.

Fix (in src/_pytest/assertion/rewrite.py):

1. visit_NamedExpr: Returns the NamedExpr inline (preserving evaluation order for function args and comparators) but references the target variable for display instead of re-evaluating the expression.
2. visit_Compare (left side): When comp.left is a NamedExpr, hoists it into a temp variable before processing comparators. This ensures comparators that reference the walrus target see the assigned value
   (fixing the assert (obj := "foo") == f(obj) case).
3. visit_Compare (comparators): For NamedExpr comparators, uses the target variable Name in results (failure message) instead of the NamedExpr node, preventing re-evaluation in the failure path. Also saves the
   left value in a temp when a walrus will overwrite it (for correct failure messages).
4. visit_BoolOp: Saves the short-circuit condition in a per-operand temp variable for the explanation path, so walrus modifications to the original variable don't corrupt the failure message.
5. visit_Assert: Clears variables_overwrite at the start of each assert, preventing stale walrus mappings from leaking between statements.

Note this PR updates the error message emitted by two tasks - I believe they present the value from before, rather than at the time the assertion fails.

TODO:

• Add additional test cases that show walrus re-evaluation in the error message path.
• Add Claude Code to the Co-authored-by commit message.

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