Fix self type specialization in overloaded __init__ methods (#384)

[officiallyutso]

Issue

Fixes #384

This PR addresses an issue with self type specialization in overloaded __init__ methods. The type checker was not properly handling specialized self types during constructor calls with generic classes.

Changes

• Modified callable_infer in callable.rs to properly handle self type substitution for __init__ method calls
• Added detection for __init__ method calls with self arguments
• Implemented proper self type substitution in return types using subst_self_type_mut
• Updated call handling to ensure correct type inference for constructor calls with specialized self types
• Integrated with typeshed stubs to ensure compatibility with standard library patterns

Testing

Verified with test cases in constructors_call_init.py, particularly those involving classes with overloaded __init__ methods that specify specialized self types (e.g., Class5[list[int]], Class5[set[str]]).

Impact

This fix ensures that when a class defines overloaded __init__ methods with specialized self types, the type checker correctly resolves constructor calls to the expected specialized type, improving type inference accuracy for generic class instantiation.

[yangdanny97]

I would expect to see some conformance changes from fixing this issue, but I don't see conformance.exp changes - did you run test.py and commit the generated files?

Also, since this feature is tricky, please add integration tests here: https://github.com/facebook/pyrefly/blob/main/pyrefly/lib/test/overload.rs

[yangdanny97]

Closing this PR due to inactivity. Feel free to reopen once you address the comments. Thanks!