Merge bug for objects with position enabled and including at least one number

[fulmicoton]

minimal reproducing test.

#[test]
fn test_positions_dynamic_mode() {
    let mut schema_builder = Schema::builder();
    let field = schema_builder.add_json_field("dynamic", TEXT);
    let schema = schema_builder.build();
    let index = Index::create_in_ram(schema.clone());
    let mut writer: IndexWriter = index.writer_for_tests().unwrap();
    let mut merge_policy = LogMergePolicy::default();
    merge_policy.set_min_num_segments(2);
    writer.set_merge_policy(Box::new(merge_policy));
    // Here a string would work.
    let doc_json = r#"{"tenant_id":75}"#;
    let vals = serde_json::from_str(doc_json).unwrap();
    let mut doc = TantivyDocument::default();
    doc.add_object(field, vals);
    writer.add_document(doc.clone()).unwrap();
    writer.commit().unwrap();
    writer.add_document(doc.clone()).unwrap();
    writer.commit().unwrap();
    writer.wait_merging_threads().unwrap();
    let reader = index.reader().unwrap();
    assert_eq!(reader.searcher().segment_readers().len(), 1);
}

[fulmicoton]

I git bisected the problem

0d4589219bcf8b4f10326e599c23ac102d28e42b is the first bad commit
commit 0d4589219bcf8b4f10326e599c23ac102d28e42b
Author: trinity-1686a <[email protected]>
Date:   Fri Oct 20 16:58:26 2023 +0200

    encode some part of posting list as -1 instead of direct values (#2185)

    * add support for delta-1 encoding posting list

    * encode term frequency minus one

    * don't emit tf for json integer terms

    * make skipreader not pub(crate) mutable

[fulmicoton]

other problem..
the spec for term freq is shaky.

for numbers, we do not record the term freq on sgement writing
but then we record a term freq of 0 after merging.

[PSeitz]

We should extend test_operation_strategy in index_writer to also include mixed types