Read trace channel_values from incremental blobs (phase 2, 3/3)

What does this MR do?

Part 3/3 of the phase-2 incremental checkpoint read path (split out of !242398 (closed)). Stacked on !247133 (merged) (the read engine); independent of and parallel to the internal-endpoint MR.

Makes trace_channel_values assemble the latest checkpoint's channel_values from the slim header + incremental blobs when duo_workflow_read_incremental_checkpoints is on, without touching the full p_duo_workflows_checkpoints table. When the flag is off it reads the full checkpoint jsonb as before.

Dependencies

• Requires !247133 (merged) (read engine + model methods + FF). Retarget to master once that merges.

Testing

trace.jsonl request specs updated and green.

Related: #604677 (closed)

---

Restructure — epic &22939

Part of the phase-2 read-path epic. Each consumer is gated by its own flag in addition to the master kill switch.

• Delivery issue: #606998
• Feature flag: dw_read_blobs_trace
• FF rollout issue: #606988

Code still gates on the shared duo_workflow_read_incremental_checkpoints flag today; repointing to the per-consumer flag is deferred until the restructure lands.