feat: expose staged index segment transactions

[ragnorc]

Summary

• Add DatasetIndexExt::build_existing_index_segments_transaction to prepare an Operation::CreateIndex transaction for existing index segments without committing.
• Refactor commit_existing_index_segments to reuse the staged transaction path.
• Add coverage that verifies staging does not publish the index and the returned transaction can be committed via CommitBuilder.

Tests

• cargo fmt --all
• cargo test -p lance existing_index_segments -- --nocapture
• cargo test -p lance test_vector_execute_uncommitted_segments_commit_without_staging -- --nocapture
• External crate cargo check against the new public API

Fixes #6666

[claude]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add a default body for new DatasetIndexExt method

Adding build_existing_index_segments_transaction as a required method on the public DatasetIndexExt trait is a breaking API change for downstream crates that implement this trait: they will fail to compile immediately after upgrading, even if they never use this new functionality. Because DatasetIndexExt is publicly exported, this should be introduced with a default implementation (or via a new extension trait) to preserve semver compatibility.

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[codecov]

Codecov Report

❌ Patch coverage is 98.55072% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	98.55%	1 Missing ⚠️

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