fix(search): wait for merge threads before dropping the bulk writer

[Azur1337]

RoomIndex::bulk_execute oepns a tantivy IndexWriter, commits, and then lets it drop without calling wait_merging_threads(). The writers background merge threads can outlive it and still be running when the next bulk_execute opens a new writer on the same index, so the two race over the same segments

During bulk indexing (e.g. backfilling a room history) this surfaces as

      tantivy::indexer::segment_manager: couldn't find segment in SegmentManager

warnings and leaves the index more fragmented than necessary

To fix this, we wait for the writers merge threads to finish before it is dropped at the end of bulk_execute. No public api change

---

• I've documented the public API changes in the appropriate changelog files (see [Writing changelog entries][pull-request-template-changelog]).
• This PR was made with the help of AI.

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.91%. Comparing base (7bf142a) to head (9cd1bbf).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-search/src/index/mod.rs	88.88%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6731      +/-   ##
==========================================
- Coverage   89.92%   89.91%   -0.01%     
==========================================
  Files         398      398              
  Lines      111157   111187      +30     
  Branches   111157   111187      +30     
==========================================
+ Hits        99954    99970      +16     
- Misses       7400     7407       +7     
- Partials     3803     3810       +7     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing Azur1337:fix/search-bulk-execute-wait-merges (9cd1bbf) with main (82a51e0)}

[Hywan]

Thanks for having worked on this, it looks good to me. Can you add a test please?

[Hywan]

I've run this test with and without your previous commit and nothing changes. I quote your description of the problem:

RoomIndex::bulk_execute oepns a tantivy IndexWriter, commits, and then lets it drop without calling wait_merging_threads(). The writers background merge threads can outlive it and still be running when the next bulk_execute opens a new writer on the same index, so the two race over the same segments

So I understand we need at least 2 bulk_execute calls. We have a single one here.