Extensions on nested generic mixins don't promote types properly

When declaring extensions on nested generic mixins `Mixin>` for example; the last type `T` is lost, and the compiler thinks it's dynamic.
leading to runtime errors when the type mismatches.
```dart
mixin TestMixin {
void operateOnValue(T value);
}
// First class expects a string.
class C1 with TestMixin {
@override
void operateOnValue(String value) {
print('operateOnValue: $value');
}
}

// Second class expects C1 which is a TestMixin
// so this class is matched with TestMixin>
class C2 with TestMixin {
@override
void operateOnValue(C1 value) {
// do nothing.
}
}

// extension targets TestMixin>.
extension NestedMixinExt>
on TestMixin {
void operateOnNestedValue(TestSub nestedMixin, TSub sub) {
print('calling operateOnValue, sub type is: $TSub');
nestedMixin.operateOnValue(sub);
}
}

void main(List arguments) {
final c1 = C1(); //TestMixin
final c2 = C2(); //TestMixin == TestMixin>

c2.operateOnNestedValue(c1, 'hello'); // prints "operateOnValue: hello" as expected.
c2.operateOnNestedValue(c1, 50); // compiles normally, and throws runtime error.
// type 'int' is not a subtype of type 'String' of 'value'
}
```
output of the program is:
```
calling operateOnValue with value hello, sub type is: dynamic
operateOnValue: hello
calling operateOnValue with value 50, sub type is: dynamic
Unhandled exception:
type 'int' is not a subtype of type 'String' of 'value'
```
also in my use case, I can't change the extension to
```dart
extension NestedMixinExt on TestMixin>
```
since I need access to the exact nested type.

dart info:
- Dart 3.1.3 (stable) (Tue Sep 26 14:25:13 2023 +0000) on "windows_x64"
- on windows / "Windows 10 Pro N" 10.0 (Build 19045)

Start with the supplied Dart reproduction and its main entry point, focusing on the NestedMixinExt declaration and the two operateOnNestedValue calls. Investigate why T is treated as dynamic; done means the nested String type is preserved and the call passing 50 is rejected rather than failing at runtime.

Written by the indexing model from the issue text.