DeepSeek V4 Pro returns thinking content as typed blocks inside the
content array rather than as a top-level reasoning_content field:
[{"type": "thinking", "thinking": "..."}, {"type": "output", ...}]
_extract_reasoning only handled content as a plain string, so the
thinking text was silently dropped. On the next turn the session was
replayed without the thinking block, causing:
HTTP 400: The content[].thinking in the thinking mode must be
passed back to the API.
Fix: when content is a list and no structured reasoning field was
found, scan for items with type=='thinking' and accumulate their
'thinking' (or 'text') value into reasoning_parts. Structured fields
(reasoning, reasoning_content, reasoning_details) still take priority
so existing provider behaviour is unchanged.
Closes#21944
68854cdcdb