feat(java/driver/jni): implement getStatistics

[lidavidm]

Closes #4360.

[Copilot]

Pull request overview

This PR enables the ADBC getStatistics() / getStatisticNames() metadata APIs in the Java JNI driver by wiring the Java AdbcConnection methods through the JNI layer to the underlying C driver implementations, and adds PostgreSQL integration coverage to validate the new bindings.

Changes:

• Implement AdbcConnection#getStatistics(...) and AdbcConnection#getStatisticNames() in JniConnection.
• Add JNI-native method declarations and loader wrappers for both APIs.
• Implement the JNI C++ wrappers that call AdbcConnectionGetStatistics / AdbcConnectionGetStatisticNames, plus add Postgres integration tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
java/driver/jni/src/main/java/org/apache/arrow/adbc/driver/jni/JniConnection.java	Adds getStatistics and getStatisticNames overrides that import native streams into an ArrowReader.
java/driver/jni/src/main/java/org/apache/arrow/adbc/driver/jni/impl/NativeAdbc.java	Declares the new native entry points for statistics APIs.
java/driver/jni/src/main/java/org/apache/arrow/adbc/driver/jni/impl/JniLoader.java	Adds safe wrapper methods around the new NativeAdbc calls.
java/driver/jni/src/main/cpp/jni_wrapper.cc	Implements JNI glue for connectionGetStatistics and connectionGetStatisticNames.
java/driver/jni-validation/src/test/java/org/apache/arrow/adbc/driver/jni/PostgresIntegrationTest.java	Adds integration tests covering both new connection APIs against PostgreSQL.

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