test(index): cover no-op optimize after compaction

[lance-gatefixer]

Summary

• extend the existing optimize_indices no-op regression through index-segment consolidation and file compaction
• assert repeated optimization preserves both the dataset version and the complete dataset file set

Root cause and fix

The original defect rebuilt steady-state index groups even when no fragments were unindexed, creating a new index UUID directory and manifest version on every call. The production no-op checks are already present on main; this change closes the remaining regression-coverage gap for the issue's reported compact-then-reoptimize sequence so future compaction or index-remap changes cannot silently restore the disk churn.

Validation

• make build
• uv run pytest python/tests/test_optimize.py::test_optimize_indices_second_call_is_noop
• uv run make format
• uv run make lint (passes; Pyright reports 10 existing missing-stub warnings)

Fixes #6659

[lance-gatekeeper]

✅ Gate recommendation: approve.

The change extends the existing no-op regression through index-segment consolidation and an actual fragment compaction while preserving both dataset-version and full file-set invariants. Reusing the existing all-index test keeps this coverage focused on the Python end-to-end contract.