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Upstream proxies (account 4, 112) return `"<nil>"` as the error.type in
their JSON responses — a Go fmt.Sprintf("%v", nil) artifact. Since
`normalizeOpsErrorType` only checked for empty string, the literal
"<nil>" passed through and poisoned the entire classification chain:
error_phase was misclassified as "internal" (instead of "request"),
severity was inflated to P2, and the stored error_type was meaningless.

Add `isKnownOpsErrorType` whitelist so any unrecognised type falls
through to the code-based or default "api_error" classification.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>