[syntax-errors] Fix multiple assignment for class keyword argument (#17184)

Summary
--

Fixes #17181. The cases being tested with multiple *keys* being equal
are actually a slightly different error, more like the error for
`MatchMapping` than like the other multiple assignment errors:

```pycon
>>> match x:
...     case Class(x=x, x=x): ...
...
  File "<python-input-249>", line 2
    case Class(x=x, x=x): ...
                      ^
SyntaxError: attribute name repeated in class pattern: x
>>> match x:
...     case {"x": 1, "x": 2}: ...
...
  File "<python-input-251>", line 2
    case {"x": 1, "x": 2}: ...
         ^^^^^^^^^^^^^^^^
SyntaxError: mapping pattern checks duplicate key ('x')
>>> match x:
...     case [x, x]: ...
...
  File "<python-input-252>", line 2
    case [x, x]: ...
             ^
SyntaxError: multiple assignments to name 'x' in pattern
```

This PR just stops the false positive reported in the issue, but I will
quickly follow it up with a new rule (or possibly combined with the
mapping rule) catching the repeated attributes separately.

Test Plan
--

New inline `test_ok` and updating the `test_err` cases to have duplicate
values instead of keys.

crates/ruff_python_parser/src/semantic_errors.rs

@@ -794,6 +794,10 @@ struct MultipleCaseAssignmentVisitor<'a, Ctx> {
 
 impl<'a, Ctx: SemanticSyntaxContext> MultipleCaseAssignmentVisitor<'a, Ctx> {
     fn visit_pattern(&mut self, pattern: &'a Pattern) {
+        // test_ok class_keyword_in_case_pattern
+        // match 2:
+        //     case Class(x=x): ...
+
         // test_err multiple_assignment_in_case_pattern
         // match 2:
         //     case [y, z, y]: ...  # MatchSequence
@@ -802,8 +806,8 @@ impl<'a, Ctx: SemanticSyntaxContext> MultipleCaseAssignmentVisitor<'a, Ctx> {
         //     case {1: x, 2: x}: ...  # MatchMapping duplicate pattern
         //     case {1: x, **x}: ...  # MatchMapping duplicate in **rest
         //     case Class(x, x): ...  # MatchClass positional
-        //     case Class(x=1, x=2): ...  # MatchClass keyword
-        //     case [x] | {1: x} | Class(x=1, x=2): ...  # MatchOr
+        //     case Class(y=x, z=x): ...  # MatchClass keyword
+        //     case [x] | {1: x} | Class(y=x, z=x): ...  # MatchOr
         //     case x as x: ...  # MatchAs
         match pattern {
             Pattern::MatchValue(_) | Pattern::MatchSingleton(_) => {}
@@ -830,7 +834,6 @@ impl<'a, Ctx: SemanticSyntaxContext> MultipleCaseAssignmentVisitor<'a, Ctx> {
                     self.visit_pattern(pattern);
                 }
                 for keyword in &arguments.keywords {
-                    self.insert(&keyword.attr);
                     self.visit_pattern(&keyword.pattern);
                 }
             }