Allow reverse operator methods on protocols

[Will-thom]

Fixes #10786.

Summary

• Skip reverse-operator signature and overlap checks for protocol definitions, matching their structural typing role rather than concrete runtime dispatch.
• Add regression coverage for a Protocol defining overloaded rpow with a modulo argument.

Tests

• ..venv\Scripts\pytest.exe mypy/test/testcheck.py::TypeCheckSuite::check-protocols.test -q
• ..venv\Scripts\pytest.exe mypy/test/testcheck.py::TypeCheckSuite::check-classes.test -q

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅