test: cover aggregation merge engine bucket modes

[QuakeWang]

Purpose

Linked issue: #257

Aggregation merge engine support existed for fixed and ordinary dynamic bucket PK tables, but the tests did not explicitly cover dynamic bucket index restore after table reload, and the docs did not describe Rust's current basic-mode support boundary. This left regressions around bucket=-1 routing and Java feature-parity assumptions easy to miss.

Brief change log

• Add a DataFusion e2e test for merge-engine=aggregation with bucket=-1, small dynamic-bucket.target-row-num, duplicate PK writes, multiple reloads, and final read validation.
• Extend the fixed-bucket aggregation e2e to use multiple buckets.
• Add a core writer test proving TableWrite::new accepts bucket=-1 + merge-engine=aggregation.
• Document Rust aggregation basic mode and unsupported options.

Tests

• cargo fmt --all -- --check
• git diff --check
• cargo test -p paimon-datafusion --test pk_tables test_pk_aggregation
• cargo test -p paimon-datafusion --test dynamic_bucket_tables aggregation
• cargo test -p paimon --lib aggregation
• cargo test -p paimon --lib table_write

API and Format

Documentation

Updated SQL docs to clarify aggregation basic-mode support and unsupported features.