Report configured NULL ordering in JDBC metadata

[hutiefang76]

The four JDBC NULL-order metadata flags currently return fixed, contradictory values (high=true and atStart=true) regardless of the connection setting. With the default NULLS_LAST, real queries put NULL last for both ASC and DESC, while metadata claims the opposite.

Read default_null_order from the connection when each flag is requested and report the corresponding JDBC ordering. The regression changes all four supported settings on one connection, observes real ASC/DESC query results, and derives the metadata expectations from those observations.

Validation: the new regression fails against the unchanged implementation. All current Java and test sources compile with JDK 17 targeting Java 8; all 33 TestMetadata tests pass against the existing DuckDB JNI build, including the new regression. Official clang-format 11.0.1 passes for both complete changed files. The native library was not rebuilt and the entire JDBC suite was not run.

AI assistance: implementation and first source review used DeepSeek; the contributor reviewed the full patch, applied review corrections, and executed the regression, metadata suite, and formatting checks.

[staticlibs]

Thanks for the PR! Looks good to me, just please rebase it on the latest main to resolve the merge conflict.