Add comprehensive tests for MultiDict.__eq__ vs different types

[Copilot]

Adds comprehensive test coverage for the MultiDict.__eq__ method against various types as requested in the issue. The new test function test_multidict_equality() validates equality comparisons with:

Test Coverage

MultiDict vs MultiDict: Tests same content, different content, and different order scenarios, including empty instances.

MultiDict vs multidict.MultiDict: Ensures compatibility with the external multidict library's MultiDict implementation, testing both matching and non-matching content/order.

MultiDict vs other MultiMappings: Tests against ListMultiDict (the minimal implementation used in tests) to verify the MultiMapping comparison logic.

MultiDict vs regular dicts: Validates the special dict comparison logic that checks all MultiDict items against dict values, including edge cases with duplicate keys and length mismatches.

MultiDict vs non-mapping types: Verifies that __eq__ correctly returns NotImplemented for non-mapping types (strings, integers, lists, None), while the == operator ultimately returns False.

Key Behaviors Tested

• Order sensitivity: MultiDict comparisons with other MultiMappings respect item order
• Length checking: Dict comparisons fail fast on length mismatch
• Duplicate key handling: Dict comparisons require ALL MultiDict items to match the corresponding dict value
• NotImplemented protocol: Proper handling of non-mapping types
• Edge cases: Empty containers, special values (None, 0, ""), KeyError scenarios

The tests ensure the equality implementation behaves correctly across all supported comparison types while maintaining backward compatibility.

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[gerlero]

@copilot Thanks!