A mid-stream SSE drop surfaces as a raw undici `TypeError: terminated`, which was classified as a non-retryable generic error and failed the turn on the first attempt. Route raw transport-layer errors through the connection-error heuristic so a dropped stream becomes a retryable APIConnectionError and is retried transparently. User aborts (ESC) are unaffected — the retry loop checks the abort signal before retrying.
Related to #149.
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