fix(java): align version type from i32 to u64

[majin1102]

This should be a mistake introduced long long ago

[github-actions]

Code Review

Summary: This PR correctly fixes a type mismatch where the Java bindings were using int/Integer for dataset versions while the underlying Rust API uses u64.

Verdict: ✅ LGTM

The changes are straightforward and correct:

1. Java ReadOptions.version changed from Optional<Integer> to Optional<Long>
2. Java Dataset.openNative parameter updated accordingly
3. JNI layer updated to use existing get_u64_opt() instead of get_int_opt()
4. Rust BlockingDataset::open version parameter changed from Option<i32> to Option<u64>

The get_u64_opt() FFI helper already exists and correctly handles the Optional<Long> to Option<u64> conversion.

Existing tests using setVersion(1) will continue to work due to Java's implicit widening from int literals to long.

No issues found.