fix(indexing): write FTS metadata even without partitions

[cmccabe]

During a distributed FTS index build, write canonical metadata even when no partitions are produced. This distinguishes a valid empty index segment from a failed or incomplete build.

[LuQQiu]

@cmccabe emmm wonder do we support search those empty partition FTS yet....
@BubbleCal do we support an FTS index segment without any partitions?

[lance-gatekeeper]

✅ Gate recommendation: approve.

The reviewed FTS fix remains unchanged, and the additional transaction-test edits use the canonical file-version resolver without altering behavior. A finalized FTS segment with no partitions is supported: its end-to-end search returns an empty result rather than an error.

[cmccabe]

@LuQQiu : we do actually test loading an empty index in test_all_empty_string_documents_build_empty_index. If there is a bug related to empty index support, I think that would just merit a follow-on change?