WhatsApp's pre-key endpoint returns 406 not-acceptable if ANY single JID
in the batch is in a broken state (deleted account, deactivated, etc.).
With Baileys' default behavior of asking for the whole participant list at
once, one stale member poisons the whole group send.
Chunk participant JIDs into batches of 5 and tolerate per-chunk failures.
The send fan-out then works for the participants whose sessions did land,
which covers the vast majority of real-world groups.
Also adds explicit pino logging so we can see which chunks failed during
diagnosis.
yiekheng
5259f88776