feat: support cleanup explain for python and java

[yanghua]

Summary

Add Java and PyLance APIs for cleanup explain.

• Add PyLance Dataset.explain_cleanup_old_versions(...) with cleanup explanation
  result types
• Export PyLance cleanup explanation types from the top-level lance package
• Add Java dataset.cleanup(policy).explain() / .execute() operation API
• Keep Java cleanupWithPolicy(policy) as a compatibility wrapper for execute
• Add JNI conversions and tests for cleanup explanations

Testing

• cargo fmt --all
• cargo fmt --manifest-path ./java/lance-jni/Cargo.toml --all
• cargo check --manifest-path python/Cargo.toml
• cargo check --manifest-path ./java/lance-jni/Cargo.toml
• ./mvnw compile
• ./mvnw -Dtest=CleanupTest test
• uv run --frozen --python 3.12 pytest python/tests/ test_dataset.py::test_explain_cleanup_old_versions