refactor(index): remove dead segment commit branch

[ddupg]

Summary

• Remove the dead segment-commit branch from CreateIndexBuilder::execute.
• Remove the BTree-only helper used solely by that branch and rename the stale IVF-RQ test.
• The branch became redundant in #6889, which replaced its segment metadata conversion with the same CreateIndex transaction as the fallback branch but left the guard in place.

Testing

• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings
• Focused index::create vector, IVF-RQ, and BTree tests

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: QUIET

Plan: Pro Plus

Run ID: d82f3a47-a965-4eae-9d91-2c3144fc5385

📥 Commits

Reviewing files that changed from the base of the PR and between 71b4515 and ef796d0.

📒 Files selected for processing (1)

• rust/lance/src/index/create.rs

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📝 Walkthrough

Walkthrough

CreateIndexBuilder::execute now always builds the create-index transaction with new_idx.dataset_version, removing segment-commit path selection and its helper functions. The related IVF-RQ test is renamed without changing its logic.

Changes

Index transaction construction

Layer / File(s)	Summary
Unify index transaction construction rust/lance/src/index/create.rs	CreateIndexBuilder::execute uses one TransactionBuilder::new path with the dataset version and transaction properties; obsolete path-selection helpers are removed and the affected test is renamed.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• lance-format/lance#6806: Changes the staged index-segment transaction API involved in index transaction handling.
• lance-format/lance#7244: Modifies CreateIndexBuilder::execute transaction-path selection in the same file.

Suggested labels: A-index

Suggested reviewers: xuanwo, jackye1995, westonpace

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: removing the redundant segment commit branch from index creation.
Description check	✅ Passed	The description matches the diff and explains the branch removal, helper cleanup, test rename, and testing performed.
Docstring Coverage	✅ Passed	Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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