feat(index): add btree global index drop

[QuakeWang]

Purpose

BTree global index already supports explicit creation, but there was no matching drop API. Once a BTree index exists, users could not remove its index manifest entries through the table/DataFusion API, which also made safe rebuild flows awkward because create rejects overlapping existing global index ranges.

Brief change log

• Add a BTree global index drop builder on Table.
• Drop matching current-snapshot index manifest entries via CommitMessage.deleted_index_files.
• Keep the operation idempotent: no snapshot, no index manifest, or no matching BTree index returns success with zero dropped entries.
• Add CALL sys.drop_global_index(table => '...', index_column => '...', index_type => 'btree').
• Reject non-BTree index types and unsupported partitions / dry_run arguments explicitly.

Tests

• cargo fmt --all -- --check
• cargo clippy --all-targets --workspace --features fulltext,vortex,mosaic -- -D warnings
• cargo test -p paimon table::btree_global_index_drop_builder --lib
• cargo test -p paimon-datafusion --test procedures drop_global_index
• cargo test -p paimon-datafusion --test procedures create_global_index
• cargo test -p paimon table::btree_global_index_build_builder --lib
• cargo check -p paimon
• cargo check -p paimon-datafusion

API and Format

Adds a Rust table API and DataFusion procedure for BTree global index drop.

Documentation

No docs update in this PR; this only exposes the narrow BTree drop lifecycle already represented by index manifest delete commits.

[JingsongLi]

+1