Validate AWS EventStream framing, header bounds, CRCs, error frames,
and terminal stop metadata before exposing Kiro output. Classify stop
reasons into dispositions (complete / retryable / terminal_incomplete /
refusal) and retry once when the stream ends with a malformed tool call,
ellipsis-only output, or a short future-action sentence.
Fail closed: propagate streaming failures as error SSE (502) instead of
collapsing them into a successful stop, so incomplete responses no longer
leak as final answers.
Detect the observed evidence-prefixed trailing progress final without
broadening the Chinese heuristic to completed findings.
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