Fix MoreLikeThis panic on indexes with deleted documents

[stumpylog]

What

MoreLikeThisQuery panics with doc_count >= doc_freq (in idf(), src/query/bm25.rs) when the searched index contains soft-deleted documents.

thread '<unnamed>' panicked at src/query/bm25.rs:53:5:
95 >= 100

Why

In MoreLikeThis::create_score_term the idf denominator is computed from SegmentReader::num_docs(), which counts alive documents only:

let num_docs = searcher
    .segment_readers()
    .iter()
    .map(|segment_reader| segment_reader.num_docs() as u64)
    .sum::<u64>();
...
let doc_freq = searcher.doc_freq(term)?;
let idf = idf(doc_freq, num_docs);

searcher.doc_freq(term) reads doc_freq straight from the term dictionary, and that value keeps counting soft-deleted documents until a merge expunges them. So after documents are deleted (e.g. a bulk import followed by deletions, with no merge in between), a common term's doc_freq can exceed the alive-only num_docs, and idf() trips its assert!(doc_count >= doc_freq).

Ordinary search does not hit this because the standard BM25 statistics provider uses Searcher::total_num_docs(), which sums SegmentReader::max_doc() (deleted documents included) — consistent with how doc_freq is counted.

Fix

Use max_doc() instead of num_docs() for the MLT idf denominator, so the document count and doc_freq are counted consistently (both include deleted documents). This mirrors total_num_docs() used by the regular BM25 scorer.

Test

Adds test_more_like_this_query_with_deleted_documents: indexes six documents sharing body terms (with NoMergePolicy so postings survive), deletes two, and runs a MoreLikeThis query against a survivor. It panics on main and passes with this change.

Context

Discovered downstream in paperless-ngx, where "More Like This" returns HTTP 500 after bulk import/delete cycles: paperless-ngx/paperless-ngx#13024

Developed with assistance from Claude Code.

[stumpylog]

I was a little confused about the released vs not state. There's a 0.26.1 tag (https://github.com/quickwit-oss/tantivy/releases/tag/0.26.1) but the header here says 0.26.0 is (Unreleased). So just let me know if this needs moving/updating.