fix(agent): extract thinking from content-list blocks for DeepSeek V4 Pro

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

run_agent.py

@@ -3529,6 +3529,19 @@ class AIAgent:
         # instead of returning structured reasoning fields.  Only fall back
         # to inline extraction when no structured reasoning was found.
         content = getattr(assistant_message, "content", None)
+        if not reasoning_parts and isinstance(content, list):
+            # DeepSeek V4 Pro (and compatible providers) return content as a
+            # list of typed blocks, e.g.:
+            #   [{"type": "thinking", "thinking": "..."}, {"type": "output", ...}]
+            # Without this branch the thinking text is silently dropped and the
+            # next turn fails with HTTP 400 ("thinking must be passed back").
+            # Refs #21944.
+            for block in content:
+                if isinstance(block, dict) and block.get("type") == "thinking":
+                    thinking_text = block.get("thinking") or block.get("text") or ""
+                    thinking_text = thinking_text.strip()
+                    if thinking_text and thinking_text not in reasoning_parts:
+                        reasoning_parts.append(thinking_text)
         if not reasoning_parts and isinstance(content, str) and content:
             inline_patterns = (
                 r"<think>(.*?)</think>",