[ty] Silence false positives for PEP-695 ParamSpec annotations (#18001)

## Summary

Suppress false positives for uses of PEP-695 `ParamSpec` in `Callable`
annotations:
```py
from typing_extensions import Callable

def f[**P](c: Callable[P, int]):
    pass
```

addresses a comment here:
https://github.com/astral-sh/ty/issues/157#issuecomment-2859284721

## Test Plan

Adapted Markdown tests

crates/ty_python_semantic/resources/mdtest/annotations/callable.md

@@ -249,10 +249,12 @@ Using a `ParamSpec` in a `Callable` annotation:
 ```py
 from typing_extensions import Callable
 
-# TODO: Not an error; remove once `ParamSpec` is supported
-# error: [invalid-type-form]
 def _[**P1](c: Callable[P1, int]):
-    reveal_type(c)  # revealed: (...) -> Unknown
+    reveal_type(P1.args)  # revealed: @Todo(ParamSpec)
+    reveal_type(P1.kwargs)  # revealed: @Todo(ParamSpec)
+
+    # TODO: Signature should be (**P1) -> int
+    reveal_type(c)  # revealed: (...) -> int
 ```
 
 And, using the legacy syntax: