Apply method signature hooks to super calls

[aryansk]

Fixes #21744

Summary

• Resolve the defining method fullname for calls through super().
• Pass the receiver type to get_method_signature_hook for super method calls.
• Add a regression to the existing custom-plugin signature-hook test.

Tests

• runtests.py check-custom-plugin.test: 64 passed.

This PR was prepared with AI assistance, as permitted by the contributor guidance, and the implementation and test results were reviewed locally.