Classify observability errors

[Gabriel-Darbord]

Summary

• Record normalized error kind, code, class, cause class, and schema violation counts/keywords on observability call records.
• Aggregate local metrics by error class, kind, and code.
• Export the same low-cardinality error attributes through the optional OpenTelemetry backend.
• Let test-only tools opt out of observability so fixture calls do not pollute telemetry.