🧪 [test] add comprehensive tests for MKCoordinateSpan

[google-labs-jules]

🎯 What: The testing gap addressed
Tests for the MKCoordinateSpan class were missing (aside from a small misplaced test group in another file). Added missing tests to fully cover MKCoordinateSpan.

📊 Coverage: What scenarios are now tested

• Value equality of MKCoordinateSpan.
• Exceptions (AssertionError) when invalid negative latitudeDelta or longitudeDelta values are provided.
• Round-tripping serialization via toPlatform() and deserialization via fromPlatform().
• toString() string representation.

✨ Result: The improvement in test coverage
Test coverage is complete for MKCoordinateSpan. mk_coordinate_region_test.dart no longer has misplaced test groups.

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PR created automatically by Jules for task 15949460858570597831 started by @esenmx

[google-labs-jules]

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