The 999 handler's rejection pass was looking up claims by patient
control number, but Gainwell rejects at the SET level (ST envelope)
when the whole batch fails the NM109=CO_TXIX rule. That meant a SET
rejection was treated as a no-op even though every claim in the SET
was actually rejected by the payer.
Add a batch_envelope_index param (mirrors apply_999_acceptances from
SP28) so SET-level rejections cascade to every claim in the SET.
Falls back to the legacy PCN lookup when the index has no entry.
Also tightens test_payer.py: PayerConfig.co_medicaid() now returns
payer_id='CO_TXIX' and payer_name='CO_TXIX' per HCPF 837P Companion
Guide (June 2025 - Version 2.5).
Nora