Add test and fix stub for sklearn.config_context

[fhoehle]

Summary

This PR adds a new test asserting that the sklearn.config_context context manager is correctly understood.
The initial failing test is fixed by correcting the corresponding sklearn.config_context type hint and adding the contextlib.contextmanager decorator.

Issue #3154 describes the observed error. The originating cause is the incorrect sklearn-stub in the pyrefly_bundled crate.

The deprecation warning regarding Iterator[None] is mitigated by replacing it with Generator

Test Plan

• Executing the new (failing) test at 03b707d7bb6aeb55d9c5f2b6dd7b6e7367fe43a4, which failed with the above-described bad-context-manager error.
  cargo test -p pyrefly --lib test_sklearn
• Successful execution at HEAD of this branch
• Successful run of test.py

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[rchen152]

Thanks for the PR! Pyrefly's bundled sklearn stubs are mirrored from here:

pyrefly/crates/pyrefly_bundled/third_party/stubs_metadata.json

Line 41 in b20fa1f

"repository": "https://github.com/microsoft/python-type-stubs",

. So rather than editing our bundled copy, it would be better to fix the stubs upstream and then bundle a fresh copy.

(If you get pushback on or lack of response to an upstream PR, we can consider doing a local patch instead. We'd need to record the patch somewhere, so we know what we've modified.)