Structured runs: sanitise persisted/emitted provider errors, not just log lines #132

From the review on #129 (deferred, UNCERTAIN). In structured-output mode, processor.ts redacts its log.error line, but the caught exception still flows through MessageV2.fromError onto the persisted assistant message and the error bus events. That has been true for every run since before #129.

Open question: can a provider's error responseBody echo the model's own rejected tool-call arguments? If any supported provider does, apply the same bounded summary (name, statusCode, isRetryable, classified message) before persistence. Terminal NDJSON errors were already sanitised by the unknown-finish retry work (#122).