Report Object class for dynamic UNION and VARIANT metadata

[hutiefang76]

UNION and VARIANT columns currently report java.lang.String from getColumnClassName even when getObject returns an Integer or another member type. Consumers that select casts from the metadata can receive a ClassCastException. Return java.lang.Object for these dynamic types so every non-null member is an instance of the reported class. The shared mapping also corrects parameter metadata; SQL type codes remain Types.OTHER.

Regression tests cover mixed integer/string/null rows, metadata before row iteration, compatibility of returned values, unchanged VARCHAR/JSON neighbors, and UNION/VARIANT parameters. Both new column regressions fail against the original class.

Using JDK 17 and the official test runner, TestMetadata passed 34 tests, TestParameterMetadata 7, TestVariant 15, and the array method filter 40. Validation compiled the three changed Java classes and ran them ahead of an existing immutable JNI build on the classpath. No native rebuild or full project suite was run.

AI assistance: DeepSeek assisted implementation and initial review. The complete diff and actual baseline/modified test output were checked before submission.

[staticlibs]

Thanks for the PR! The change looks good to me, just please adjust the test TestDuckDBJDBC#test_getColumnClassName that started failing after the change.