The POST /contact/v3/users/basic_batch endpoint caps user_ids at 1~10
per request, but batchResolveByBasicContact was chunking by 50. When
user identity needed to resolve >10 unresolved sender names, the
single oversized request was rejected, causing the batch resolver to
bail out and leave sender names empty for the rest.
Lower batchSize to 10 and add a unit test that exercises 25 missing
IDs and asserts they are sent as 10 / 10 / 5.
