[ty] Substitute for typing.Self when checking protocol members (#21569)

This patch updates our protocol assignability checks to substitute for
any occurrences of `typing.Self` in method signatures, replacing it with
the class being checked for assignability against the protocol.

This requires a new helper method on signatures, `apply_self`, which
substitutes occurrences of `typing.Self` _without_ binding the `self`
parameter.

We also update the `try_upcast_to_callable` method. Before, it would
return a `Type`, since certain types upcast to a _union_ of callables,
not to a single callable. However, even in that case, we know that every
element of the union is a callable. We now return a vector of
`CallableType`. (Actually a smallvec to handle the most common case of a
single callable; and wrapped in a new type so that we can provide helper
methods.) If there is more than one element in the result, it represents
a union of callables. This lets callers get at the `CallableType`
instances in a more type-safe way. (This makes it easier for our
protocol checking code to call the new `apply_self` helper.) We also
provide an `into_type` method so that callers that really do want a
`Type` can get the original result easily.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -2003,6 +2003,7 @@ python-version = "3.12"
 ```
 
 ```py
+from typing import final
 from typing_extensions import TypeVar, Self, Protocol
 from ty_extensions import is_equivalent_to, static_assert, is_assignable_to, is_subtype_of
 
@@ -2094,6 +2095,13 @@ class NominalReturningSelfNotGeneric:
     def g(self) -> "NominalReturningSelfNotGeneric":
         return self
 
+@final
+class Other: ...
+
+class NominalReturningOtherClass:
+    def g(self) -> Other:
+        raise NotImplementedError
+
 # TODO: should pass
 static_assert(is_equivalent_to(LegacyFunctionScoped, NewStyleFunctionScoped))  # error: [static-assert-error]
 
@@ -2112,8 +2120,7 @@ static_assert(not is_assignable_to(NominalLegacy, UsesSelf))
 static_assert(not is_assignable_to(NominalWithSelf, NewStyleFunctionScoped))
 static_assert(not is_assignable_to(NominalWithSelf, LegacyFunctionScoped))
 static_assert(is_assignable_to(NominalWithSelf, UsesSelf))
-# TODO: should pass
-static_assert(is_subtype_of(NominalWithSelf, UsesSelf))  # error: [static-assert-error]
+static_assert(is_subtype_of(NominalWithSelf, UsesSelf))
 
 # TODO: these should pass
 static_assert(not is_assignable_to(NominalNotGeneric, NewStyleFunctionScoped))  # error: [static-assert-error]
@@ -2126,6 +2133,8 @@ static_assert(not is_assignable_to(NominalReturningSelfNotGeneric, LegacyFunctio
 # TODO: should pass
 static_assert(not is_assignable_to(NominalReturningSelfNotGeneric, UsesSelf))  # error: [static-assert-error]
 
+static_assert(not is_assignable_to(NominalReturningOtherClass, UsesSelf))
+
 # These test cases are taken from the typing conformance suite:
 class ShapeProtocolImplicitSelf(Protocol):
     def set_scale(self, scale: float) -> Self: ...