After a prewalk hand-off plus mid-run compaction, the openai-responses input
builder re-encoded replayed assistant turns via convertResponsesAssistantMessage,
demoting their reasoning to <think> output_text and emitting no reasoning item.
DeepSeek (opencode-go) then rejected the thinking-mode continuation with
400 "The reasoning_text in the thinking mode must be passed back to the API"
and OMP looped on the unchanged request.
The encoder now synthesizes a reasoning_text reasoning item for every replayed
assistant turn when the target requires reasoning replay in thinking mode
(requiresReasoningContentForAllAssistantTurns / requiresReasoningContentForToolCalls),
carrying surviving thinking text when present, mirroring the chat-completions
reasoning_content safety net. Gated on reasoning being active for the request,
so non-DeepSeek Responses targets and reasoning-disabled turns are unaffected.
Fixes#8248
