fix(table): enable global index by default

[XiaoHongbo-Hope]

Summary

Align the Rust global-index.enabled default with Java Paimon and PyPaimon. When the option is absent, Rust scans can now use existing global indexes; an explicit false still disables them.

Changes

• Default global-index.enabled to true when it is not configured.
• Cover the default and explicit true / false overrides at the option layer.
• Exercise the regular table-scan path without an explicit override and verify that it reads the committed BTree index row ranges.
• Update the SQL option reference with the corrected default.

Testing

• cargo test --locked -p paimon --lib (1800 passed, 1 ignored)
• cargo test --locked -p paimon --lib table::btree_global_index_build_builder::tests (23 passed)
• Parent/head regression check: parent returns no row ranges; this PR returns [0,0] and [2,2]
• cargo fmt --all -- --check
• cargo clippy --locked -p paimon --all-targets -- -D warnings

[QuakeWang]

+1