[ty] Track different uses of legacy typevars, including context when rendering typevars (#19604)

This PR introduces a few related changes:

- We now keep track of each time a legacy typevar is bound in a
different generic context (e.g. class, function), and internally create
a new `TypeVarInstance` for each usage. This means the rest of the code
can now assume that salsa-equivalent `TypeVarInstance`s refer to the
same typevar, even taking into account that legacy typevars can be used
more than once.

- We also go ahead and track the binding context of PEP 695 typevars.
That's _much_ easier to track since we have the binding context right
there during type inference.

- With that in place, we can now include the name of the binding context
when rendering typevars (e.g. `T@f` instead of `T`)

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

@@ -16,7 +16,7 @@ from typing import Self
 
 class Shape:
     def set_scale(self: Self, scale: float) -> Self:
-        reveal_type(self)  # revealed: Self
+        reveal_type(self)  # revealed: Self@Shape
         return self
 
     def nested_type(self: Self) -> list[Self]:
@@ -24,7 +24,7 @@ class Shape:
 
     def nested_func(self: Self) -> Self:
         def inner() -> Self:
-            reveal_type(self)  # revealed: Self
+            reveal_type(self)  # revealed: Self@Shape
             return self
         return inner()
 
@@ -38,13 +38,13 @@ reveal_type(Shape().nested_func())  # revealed: Shape
 
 class Circle(Shape):
     def set_scale(self: Self, scale: float) -> Self:
-        reveal_type(self)  # revealed: Self
+        reveal_type(self)  # revealed: Self@Circle
         return self
 
 class Outer:
     class Inner:
         def foo(self: Self) -> Self:
-            reveal_type(self)  # revealed: Self
+            reveal_type(self)  # revealed: Self@Inner
             return self
 ```
 
@@ -151,7 +151,7 @@ from typing import Self
 
 class Shape:
     def union(self: Self, other: Self | None):
-        reveal_type(other)  # revealed: Self | None
+        reveal_type(other)  # revealed: Self@Shape | None
         return self
 ```
 

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

@@ -26,7 +26,7 @@ def i(callback: Callable[Concatenate[int, P], R_co], *args: P.args, **kwargs: P.
 
 class Foo:
     def method(self, x: Self):
-        reveal_type(x)  # revealed: Self
+        reveal_type(x)  # revealed: Self@Foo
 ```
 
 ## Type expressions