Get Agent Observability Evaluator Trends

Evaluator metrics to chart. Omitted means use the org's saved dashboard config.

One section's worth of cards plus the metadata describing how complete it is.

Availability semantics are frozen:

• at least one card succeeded → available=True; data_status is complete when every card succeeded and partial when some did not.
• every card failed → available=False, data_status=unavailable, and section-level degraded is set from the most common card failure. Cards remain in the payload with their own degraded payloads.