fix(localagent): stop re-seeding stream accumulator with previous round content (#2329)

When the LLM (e.g. MiniMax-M3) returns multiple rounds after tool calls, the _StreamAccumulator was initialized with initial_content=first_content, causing every subsequent round to repeat the entire first message. Remove the re-seeding so each round starts with a clean accumulator.

Add regression test verifying multi-round tool call content is not duplicated.

src/langbot/pkg/provider/runners/localagent.py

@@ -523,7 +523,6 @@ class LocalAgentRunner(runner.RequestRunner):
             final_msg = stream_accumulator.final_message()
 
         pending_tool_calls = final_msg.tool_calls
-        first_content = final_msg.content
         if isinstance(final_msg, provider_message.MessageChunk):
             first_end_sequence = final_msg.msg_sequence
 
@@ -606,9 +605,13 @@ class LocalAgentRunner(runner.RequestRunner):
             )
 
             if is_stream:
+                # Do NOT re-seed the accumulator with first_content:
+                # the previous round's text was already pushed to the
+                # platform adapter. Re-seeding would cause every
+                # subsequent round to repeat the entire opening line,
+                # which the platform then forwards as a duplicate message.
                 stream_accumulator = _StreamAccumulator(
                     msg_sequence=first_end_sequence,
-                    initial_content=first_content,
                     remove_think=remove_think,
                 )