Escalation reasons

FallbackRequest.reason and the escalate trace event carry one of these.

Reason	When
no-model	No model was configured
model-unavailable	Apple Intelligence is unavailable, disabled, or its models are not ready
too-many-steps	The step cap was reached without an answer
repeated-tool-call	The same tool was called with the same input twice
answer-invalid	Structured answer failed JSON parsing or schema validation after one retry
empty-answer	The answer was empty after one nudged retry
plan-invalid	Two malformed plans in a row
conductor-asked	The conductor returned kind: "escalate"
tool-args-invalid	Two malformed argument objects in a row
tool-error	Two tool errors in a row
guardrail	Apple’s guardrail rejected the text. Ends on device; never sent to the cloud
model-error	Refusal, overflow, or another provider error. Never retried

Three model-error turns in a row set sessionFallback.