fix(fts): derive empty segment codec from format

[BubbleCal]

What is the bug?

A distributed FTS rebuild can legitimately produce an intermediate segment with zero partitions when its assigned fragments contain no indexable tokens. InvertedIndex::posting_tail_codec currently falls back to the default VarintDelta codec for such a segment, even when its declared format is V1 / Fixed32.

Merging that empty V1 segment with a populated V1 segment then fails with:

cannot merge inverted index segments with different posting tail codecs

This is the same-format empty-segment failure observed in ENT-2323. It is narrower than the cross-format behavior proposed in #8681.

What issues or incorrect behavior does the bug cause?

V1 distributed FTS rebuilds fail during merge_existing_index_segments before the replacement index can be committed. Automated maintenance can retry the same deterministic failure.

How does this PR fix the problem?

When an inverted index has no partitions, derive its posting-tail codec from the segment's declared FTS format instead of using the global codec default. Populated segments continue to use their physical partition metadata, and cross-format merge behavior is unchanged.

Tests

Adds a regression test that merges an empty V1 segment with a populated V1 segment in both input orders. It verifies that:

• the empty segment resolves to Fixed32
• the merged index remains V1
• the populated document remains searchable

Local compilation and tests were intentionally skipped; CI is the validation path for this PR.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The empty-segment codec now follows the segment’s declared FTS format, which fixes V1 consolidation without weakening physical-codec or cross-format validation. The regression covers both segment orders and verifies the merged V1 index remains searchable.