Restrict type of AssignmentExpr.target to NameExpr (#18714)

Assignment expression targets can only be identifiers. From the
[grammar](https://docs.python.org/3.14/reference/grammar.html):

```
assignment_expression:
    | NAME ':=' ~ expression 
```

This
[corresponds](https://github.com/python/typeshed/blob/ac8f2632ec37bb4a82ade0906e6ce9bdb33883d3/stdlib/ast.pyi#L834-L837)
to the standard library AST node's `target` type:

```python
class NamedExpr(expr):
    if sys.version_info >= (3, 10):
        __match_args__ = ("target", "value")
    target: Name
```

Author: bzoracler

mypy/nodes.py

@@ -2075,7 +2075,7 @@ class AssignmentExpr(Expression):
 
     __match_args__ = ("target", "value")
 
-    def __init__(self, target: Expression, value: Expression) -> None:
+    def __init__(self, target: NameExpr, value: Expression) -> None:
         super().__init__()
         self.target = target
         self.value = value

mypy/treetransform.py

@@ -559,7 +559,7 @@ def visit_super_expr(self, node: SuperExpr) -> SuperExpr:
         return new
 
     def visit_assignment_expr(self, node: AssignmentExpr) -> AssignmentExpr:
-        return AssignmentExpr(self.expr(node.target), self.expr(node.value))
+        return AssignmentExpr(self.duplicate_name(node.target), self.expr(node.value))
 
     def visit_unary_expr(self, node: UnaryExpr) -> UnaryExpr:
         new = UnaryExpr(node.op, self.expr(node.expr))