Recall follow-up stream parity

The recall follow-up stream flag must always match its consumer. Use SSE accumulation for streaming continuations and Codex, forcing Codex streams even when a buffered result is needed; ordinary Responses and Chat may use JSON. Validate accumulation capability, including parseSSE, before forwarding—never send an unaccumulatable request. Headerless Codex SSE may be identified only through a bounded replay-preserving sniff.