fix: keep delete-by-source fast path with scalar indexes

[Xuanwo]

This teaches merge_insert to keep the delete-by-source fast path even when a scalar index exists on the join key. The actual indexed join path is still only used when unmatched target rows are kept, so the presence of index metadata should not force these operations back to the legacy full-join path.

This also adds regression coverage for full-schema FixedSizeList merges with when_not_matched_by_source(Delete) both with and without a scalar index. That closes the gap behind #6195 and preserves the earlier fix for lancedb/lancedb#3094.

[codecov]

Codecov Report

❌ Patch coverage is 99.30070% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	99.30%	0 Missing and 1 partial ⚠️

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[BubbleCal]

Nice work!