feat(java): expose index segment sizes

[majin1102]

Summary

• expose per-segment index file size through Index#getSizeBytes()
• expose aggregate logical-index size through IndexDescription#getTotalSizeBytes()
• preserve Optional.empty() for indices created before file-size tracking and retain the existing public constructor

Why

Rust and Python already expose persisted index file-size metadata, while Java drops it at the JNI boundary. Java engines therefore cannot inspect physical segment sizes when planning index maintenance, including the size-tiered work tracked in lance-spark#744.

Validation

• ./mvnw -Dtest=DatasetTest#testDescribeIndicesByName test
• ./mvnw spotless:check
• cargo clippy --target-dir target/rust-maven-plugin/lance-jni --tests --manifest-path ./lance-jni/Cargo.toml -- -D warnings

[lance-gatekeeper]

✅ Gate recommendation: approve.

This closes the Java binding gap at the existing metadata boundary: per-segment sizes remain attached to Index, aggregate size follows the Rust IndexDescription contract, and legacy unknown sizes stay optional. The thin JNI mapping is preferable to storage listing or a parallel UUID-to-size structure because it reuses metadata already loaded and avoids an extra source of truth.