Fix: Skip bad-return validation for Protocol methods (#1916)

[AryanBagade]

Summary

Protocol methods with only a docstring (no ... or implementation body) were incorrectly emitting "Function declared to return X but is missing an explicit return" errors. This is valid Python for Protocol definitions and is accepted by mypy and other type checkers.

Changes:

• Added class_key field to ReturnTypeKind::ShouldValidateAnnotation to track the enclosing class
• In solve.rs, check if the class is a Protocol and skip return validation for Protocol methods
• Added regression test covering the reported case

Fixes #1916

Test Plan

• Added test_protocol_method_with_docstring test case covering:
  • @property with docstring only
  • Regular method with docstring only
  • Method with pass
  • Method with ellipsis (already worked)
• Verified regular (non-Protocol) classes still error correctly
• Ran python3 test.py - all tests pass
• Ran cargo test --lib protocol:: - all 37 protocol tests pass

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

urllib3 (https://github.com/urllib3/urllib3)
- ERROR src/urllib3/_base_connection.py:98:32-36: Function declared to return `bool` but is missing an explicit `return` [bad-return]
- ERROR src/urllib3/_base_connection.py:105:35-39: Function declared to return `bool` but is missing an explicit `return` [bad-return]
- ERROR src/urllib3/_base_connection.py:109:45-49: Function declared to return `bool` but is missing an explicit `return` [bad-return]

cibuildwheel (https://github.com/pypa/cibuildwheel)
- ERROR cibuildwheel/environment.py:56:10-13: Function declared to return `str` but is missing an explicit `return` [bad-return]

dulwich (https://github.com/dulwich/dulwich)
- ERROR dulwich/object_store.py:317:27-81: Function declared to return `tuple[BytesIO, () -> None, () -> None]` but is missing an explicit `return` [bad-return]
- ERROR dulwich/pack.py:216:49-53: Function declared to return `bool` but is missing an explicit `return` [bad-return]
- ERROR dulwich/pack.py:219:62-69: Function declared to return `ShaFile` but is missing an explicit `return` [bad-return]

hydpy (https://github.com/hydpy-dev/hydpy)
- ERROR hydpy/auxs/calibtools.py:70:27-32: Function declared to return `float` but is missing an explicit `return` [bad-return]

tornado (https://github.com/tornadoweb/tornado)
- ERROR tornado/ioloop.py:61:25-28: Function declared to return `int` but is missing an explicit `return` [bad-return]
- ERROR tornado/platform/asyncio.py:58:25-28: Function declared to return `int` but is missing an explicit `return` [bad-return]
- ERROR tornado/websocket.py:61:44-49: Function declared to return `bytes` but is missing an explicit `return` [bad-return]
- ERROR tornado/websocket.py:64:39-44: Function declared to return `bytes` but is missing an explicit `return` [bad-return]
- ERROR tornado/websocket.py:69:38-43: Function declared to return `bytes` but is missing an explicit `return` [bad-return]
- ERROR tornado/websocket.py:72:63-68: Function declared to return `bytes` but is missing an explicit `return` [bad-return]

rotki (https://github.com/rotki/rotki)
- ERROR rotkehlchen/chain/evm/accounting/structures.py:21:10-13: Function declared to return `int` but is missing an explicit `return` [bad-return]

streamlit (https://github.com/streamlit/streamlit)
- ERROR lib/streamlit/runtime/stats.py:44:30-33: Function declared to return `str` but is missing an explicit `return` [bad-return]
- ERROR lib/streamlit/runtime/stats.py:49:23-26: Function declared to return `str` but is missing an explicit `return` [bad-return]
- ERROR lib/streamlit/runtime/stats.py:54:23-26: Function declared to return `str` but is missing an explicit `return` [bad-return]
- ERROR lib/streamlit/runtime/stats.py:59:23-26: Function declared to return `str` but is missing an explicit `return` [bad-return]
- ERROR lib/streamlit/runtime/stats.py:63:32-35: Function declared to return `str` but is missing an explicit `return` [bad-return]
- ERROR lib/streamlit/runtime/stats.py:270:33-46: Function declared to return `Sequence[str]` but is missing an explicit `return` [bad-return]

django-test-migrations (https://github.com/wemake-services/django-test-migrations)
- ERROR django_test_migrations/contrib/pytest_plugin.py:39:62-70: Function declared to return `Migrator` but is missing an explicit `return` [bad-return]

setuptools (https://github.com/pypa/setuptools)
- ERROR setuptools/_vendor/importlib_metadata/_meta.py:41:23-55: Function declared to return `dict[str, list[str] | str]` but is missing an explicit `return` [bad-return]

mypy (https://github.com/python/mypy)
- ERROR mypyc/test-data/fixtures/ir.py:18:26-30: Function declared to return `T_co` but is missing an explicit `return` [bad-return]

scrapy (https://github.com/scrapy/scrapy)
- ERROR scrapy/extensions/feedexport.py:116:39-48: Function declared to return `IO[bytes]` but is missing an explicit `return` [bad-return]
- ERROR scrapy/spiderloader.py:35:55-59: Function declared to return `Self@SpiderLoaderProtocol` but is missing an explicit `return` [bad-return]
- ERROR scrapy/spiderloader.py:38:41-53: Function declared to return `type[Spider]` but is missing an explicit `return` [bad-return]
- ERROR scrapy/spiderloader.py:42:23-32: Function declared to return `list[str]` but is missing an explicit `return` [bad-return]

[samwgoldman]

Thank you!

I think we can do this a bit more simply, by instead adjusting the logic in BindingsBuilder::function_body, where we determine the stub_or_impl value. The class key is already available in that code, so we can also avoid threading it through the binding.

For what it's worth, it looks like we always treat ... as a stub. We did this to match pyright & because the conformance tests are written this way. The conformance tests were recently updated to avoid the need to accept ... always (python/typing#2134). For now, I think we should leave this behavior alone, but might be an interesting follow-up.