feat: add python and java binding for aggregate

[jackye1995]

Follow up after #5911

[github-actions]

Code Review Summary

This PR adds Python and Java bindings for the aggregate functionality introduced in #5911. The implementation follows the existing patterns established for similar features (e.g., substrait_filter).

No Critical Issues Found

The code is straightforward and follows existing patterns well:

• Java: Adds substrait_aggregate parameter through the JNI layer using AggregateExpr::substrait()
• Python: Adds substrait_aggregate builder method and passes it to the Rust scanner

Minor Observations (Not Blocking)

1. Test Coverage: The PR adds no tests for the new bindings. However, since:

   • The core Rust functionality is thoroughly tested in feat: support aggregate in scanner #5911
   • The bindings are thin wrappers that follow the exact same pattern as substrait_filter (which also lacks binding-level tests)
   • This is consistent with the project's existing testing approach for such bindings

   This is acceptable for a follow-up binding PR.

2. Type stub completeness: The __init__.pyi file also adds stubs for other missing parameters (scan_stats_callback, strict_batch_size, order_by, disable_scoring_autoprojection) which is a nice cleanup beyond just the aggregate feature.

LGTM 👍