Check self type variable bounds for overloads

[thesammyagent]

Fixes #21646.

Summary

• re-check inferred explicit self type variables against their bounds when filtering overload items by receiver type
• add a focused regression for list[int | None].sort() and [2, None].sort()
• keep key-based sorting and comparable zero-argument sorting accepted

Tests

Automated local verification ran:

• PATH="$PWD/.venv/bin:$PATH" ./.venv/bin/python runtests.py check-selftype.test check-overloading.test
  • check-selftype.test: 120 passed
  • check-overloading.test: 269 passed, 1 skipped
• PATH="$PWD/.venv/bin:$PATH" pytest mypy/test/testcheck.py::TypeCheckSuite::check-selftype.test mypy/test/testcheck.py::TypeCheckSuite::check-overloading.test
  • 389 passed, 1 skipped
• git diff --check
• git diff --cached --check

AI assistance disclosure

This PR was produced by an automated AI-agent workflow. The workflow investigated the issue, drafted the focused patch and regression test, ran automated local verification, and performed automated review passes before submission.

Workflow provenance: the AI-agent workflow/software is created and operated by @thesammygit.