[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/resources/inline/ok/class_keyword_in_case_pattern.py

@@ -0,0 +1,2 @@
+match 2:
+    case Class(x=x): ...