Capture persistent instructions in headless Pipecat adapters

A headless Pipecat adapter can execute the correct handlers while sending an incomplete prompt to its decision model. Flow managers may deliver role instructions through LLMUpdateSettingsFrame.delta.system_instruction rather than ordinary message frames. Ignoring settings frames as transport noise can therefore remove the entire active-node router prompt. Capture that instruction, retain it across sampling-only updates, and include it when serializing the backend request. Verify the complete serialized prompt against the canonical builder after a real node transition; tests that force tool choices can otherwise pass while hiding the loss.

Applies when: A Pipecat flow is adapted to a different model API or non-pipeline transport. Keywords: Pipecat adapters system instructions settings frames prompt parity tools