[ty] Split `ParamSpec` mdtests to separate legacy and PEP 695 tests (#21687)

## Summary

This is another small refactor for
https://github.com/astral-sh/ruff/pull/21445 that splits the single
`paramspec.md` into `generics/legacy/paramspec.md` and
`generics/pep695/paramspec.md`.

## Test Plan

Make sure that all mdtests pass.

crates/ty_python_semantic/resources/mdtest/generics/legacy/paramspec.md

@@ -1,4 +1,4 @@
-# `ParamSpec`
+# Legacy `ParamSpec`
 
 ## Definition
 
@@ -115,59 +115,3 @@ P = ParamSpec("P", default=[A, B])
 class A: ...
 class B: ...
 ```
-
-### PEP 695
-
-```toml
-[environment]
-python-version = "3.12"
-```
-
-#### Valid
-
-```py
-def foo1[**P]() -> None:
-    reveal_type(P)  # revealed: typing.ParamSpec
-
-def foo2[**P = ...]() -> None:
-    reveal_type(P)  # revealed: typing.ParamSpec
-
-def foo3[**P = [int, str]]() -> None:
-    reveal_type(P)  # revealed: typing.ParamSpec
-
-def foo4[**P, **Q = P]():
-    reveal_type(P)  # revealed: typing.ParamSpec
-    reveal_type(Q)  # revealed: typing.ParamSpec
-```
-
-#### Invalid
-
-ParamSpec, when defined using the new syntax, does not allow defining bounds or constraints.
-
-This results in a lot of syntax errors mainly because the AST doesn't accept them in this position.
-The parser could do a better job in recovering from these errors.
-
-<!-- blacken-docs:off -->
-
-```py
-# error: [invalid-syntax]
-# error: [invalid-syntax]
-# error: [invalid-syntax]
-# error: [invalid-syntax]
-# error: [invalid-syntax]
-# error: [invalid-syntax]
-def foo[**P: int]() -> None:
-    # error: [invalid-syntax]
-    # error: [invalid-syntax]
-    pass
-```
-
-<!-- blacken-docs:on -->
-
-#### Invalid default
-
-```py
-# error: [invalid-paramspec]
-def foo[**P = int]() -> None:
-    pass
-```

crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md

@@ -0,0 +1,64 @@
+# PEP 695 `ParamSpec`
+
+`ParamSpec` was introduced in Python 3.12 while the support for specifying defaults was added in
+Python 3.13.
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+## Definition
+
+```py
+def foo1[**P]() -> None:
+    reveal_type(P)  # revealed: typing.ParamSpec
+```
+
+## Bounds and constraints
+
+`ParamSpec`, when defined using the new syntax, does not allow defining bounds or constraints.
+
+TODO: This results in a lot of syntax errors mainly because the AST doesn't accept them in this
+position. The parser could do a better job in recovering from these errors.
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+# error: [invalid-syntax]
+# error: [invalid-syntax]
+# error: [invalid-syntax]
+# error: [invalid-syntax]
+# error: [invalid-syntax]
+def foo[**P: int]() -> None:
+    # error: [invalid-syntax]
+    # error: [invalid-syntax]
+    pass
+```
+
+<!-- blacken-docs:on -->
+
+## Default
+
+The default value for a `ParamSpec` can be either a list of types, `...`, or another `ParamSpec`.
+
+```py
+def foo2[**P = ...]() -> None:
+    reveal_type(P)  # revealed: typing.ParamSpec
+
+def foo3[**P = [int, str]]() -> None:
+    reveal_type(P)  # revealed: typing.ParamSpec
+
+def foo4[**P, **Q = P]():
+    reveal_type(P)  # revealed: typing.ParamSpec
+    reveal_type(Q)  # revealed: typing.ParamSpec
+```
+
+Other values are invalid.
+
+```py
+# error: [invalid-paramspec]
+def foo[**P = int]() -> None:
+    pass
+```