Add test that asserts multidict.MultiDict implements abc.MutableMultiMapping

[Copilot]

This PR adds a focused test to verify that multidict.MultiDict correctly implements the MutableMultiMapping abstract base class through the existing ABC registration mechanism.

Problem

While the codebase already registers multidict.MultiDict with MutableMultiMapping via MutableMultiMapping.register(multidict.MultiDict) in multicollections/abc.py, there was no explicit test validating that this registration works correctly.

Solution

Added test_multidict_implements_mutable_multi_mapping() which:

• Creates a multidict.MultiDict instance with sample data
• Uses isinstance() to verify it's recognized as a MutableMultiMapping
• Provides a clear assertion that validates the ABC registration

def test_multidict_implements_mutable_multi_mapping() -> None:
    """Test that multidict.MultiDict implements abc.MutableMultiMapping."""
    md = multidict.MultiDict([("a", 1), ("b", 2)])
    assert isinstance(md, MutableMultiMapping)

Testing

The test was validated by:

1. Confirming it passes with the current registration in place
2. Temporarily commenting out the registration to verify the test correctly fails
3. Restoring the registration to confirm the test passes again
4. Running the full test suite (122 tests) to ensure no regressions

This change provides explicit validation of the ABC implementation relationship that was previously only implicit through the registration code.

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[gerlero]

@copilot Thanks!