Fix listDeserializer casting issue in firebase_data_connect

[google-labs-jules[bot]]

Fixed a bug in firebase_data_connect where listDeserializer would throw a TypeError when processing JSON lists because of an incorrect cast from List<dynamic> to List<T>. Modified listDeserializer and listSerializer in optional.dart to accept List<dynamic> inputs. Added regression tests in test/src/optional_regression_test.dart to verify the fix.

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PR created automatically by Jules for task 6001879865263701345 started by @maneesht