fix(index): skip scalar index rewrite when no unindexed fragments

[zhangyue19921010]

Summary

When optimize_indices was called on a scalar index in steady state (no unindexed fragments, and num_indices_to_merge <=1 ), the scalar branch of merge_indices_with_unindexed_frags still rebuilt the segment and assigned it a new UUID, churning the dataset version on every call. But in fact, the index content remains unchanged at all.

This adds the same no-op short-circuit that already exists for the Inverted branch: when unindexed is empty and num_indices_to_merge resolves to <= 1, return Ok(None) and skip the rewrite.

Behavior change

• OptimizeOptions::default() / append() / merge(1) with no pending fragments is now a true no-op
  (UUID and manifest version are preserved).

Same as the logic in inverted index

        match index_type {
            IndexType::Inverted => {
                let num_to_merge = options
                    .num_indices_to_merge
                    .unwrap_or(1)
                    .min(old_indices.len());
                if unindexed.is_empty() && num_to_merge <= 1 {
                    return Ok(None);
                }

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