fix(subagent): preserve reasoning fields across tool turns

Share assistant message construction between the main agent and subagents, and add a regression test to keep reasoning_content and thinking_blocks in follow-up tool rounds.

nanobot/agent/subagent.py

@@ -16,6 +16,7 @@ from nanobot.bus.events import InboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.config.schema import ExecToolConfig
 from nanobot.providers.base import LLMProvider
+from nanobot.utils.helpers import build_assistant_message
 
 
 class SubagentManager:
@@ -133,7 +134,6 @@ class SubagentManager:
                 )
 
                 if response.has_tool_calls:
-                    # Add assistant message with tool calls
                     tool_call_dicts = [
                         {
                             "id": tc.id,
@@ -145,19 +145,12 @@ class SubagentManager:
                         }
                         for tc in response.tool_calls
                     ]
-                    assistant_msg: dict[str, Any] = {
-                        "role": "assistant",
-                        "content": response.content or "",
-                        "tool_calls": tool_call_dicts,
-                    }
-                    # Preserve reasoning_content for providers that require it
-                    # (e.g. Deepseek Reasoner mandates this field on every
-                    # assistant message when thinking mode is active).
-                    if response.reasoning_content is not None:
-                        assistant_msg["reasoning_content"] = response.reasoning_content
-                    if response.thinking_blocks:
-                        assistant_msg["thinking_blocks"] = response.thinking_blocks
-                    messages.append(assistant_msg)
+                    messages.append(build_assistant_message(
+                        response.content or "",
+                        tool_calls=tool_call_dicts,
+                        reasoning_content=response.reasoning_content,
+                        thinking_blocks=response.thinking_blocks,
+                    ))
 
                     # Execute tools
                     for tool_call in response.tool_calls: