feat: add a fast dataset version ID API

[BubbleCal]

Summary

• add Dataset::version_id() in Rust to return the checked-out manifest version without building full version metadata
• route Python dataset.version and default checkout refs through the new fast path
• route Java Dataset.version() through a new JNI version-id accessor while keeping getVersion() unchanged
• extend Rust, Python, and Java tests to cover current, updated, and historical version reads

Testing

• cargo test -p lance test_version_id_fast_path
• cargo check --manifest-path java/lance-jni/Cargo.toml
• cargo check --manifest-path python/Cargo.toml
• Python pytest target not run in this environment (pytest unavailable)
• Java Maven test target not run in this environment (JRE unavailable)

[github-actions]

ACTION NEEDED
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[github-actions]

PR Review

Clean, well-scoped change. The fast path avoids constructing the full Version (which walks manifest fragments) for a simple ID read — good optimization.

No blocking issues found. Tests cover current, updated, and historical version reads across all three languages. JNI pattern is consistent with existing nativeGetLatestVersionId. Python bindings correctly route both version() and default checkout ref through the new path.

One minor note (non-blocking): version_id() returns u64 which is cast to jlong (i64) in JNI — this matches the existing convention throughout the file, so it's fine, but worth being aware of if version IDs ever approach i64::MAX.

LGTM.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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