fix: test_successful_wait_for_connection test: make HashableMock thread-safe to fix flaky Windows CI

[mykaul]

Summary

Fixes flaky test_successful_wait_for_connection failure on Windows CI:

TypeError: __hash__ method should return an integer

at cassandra/pool.py:582 (connection in self._trash).

Root cause

HashableMock subclasses NonCallableMagicMock and overrides __hash__ to return id(self). However, MagicMixin.__init__ replaces __hash__ on the type with a MagicMock object before any test runs — making the original override dead code that never executes.

The MagicMock standing in for __hash__ happens to return a hash-compatible value in single-threaded use, but MagicMock.__call__ is not thread-safe. In test_successful_wait_for_connection, two threads call pool.return_connection() on the same HashableMock concurrently, which triggers hash() via connection in self._trash (a set membership check). Under Windows thread scheduling, the concurrent MagicMock.__call__ can return a non-integer, causing the TypeError.

Fix

Restore a plain function as the class-level __hash__ in HashableMock.__init__, after MagicMixin has finished its work. This ensures hash() always resolves to a real, thread-safe function returning id(self).

Verification

• All 10 pool tests pass
• test_successful_wait_for_connection passes 100/100 runs in a loop

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.