fix(backend/copilot): add conversation_turn to Langfuse trace metadata

Add conversation_turn field to trace metadata with the correct turn number
based on user message count. This fixes the Langfuse observability issue where
SDK's num_turns always showed '1' when using --resume (which creates a new CLI
session each turn).

Now Langfuse traces will include:
- langsmith.metadata.num_turns: '1' (from SDK, represents current CLI session)
- metadata.conversation_turn: <actual turn> (from our code, represents conversation history)

This gives us accurate turn tracking for multi-turn conversation analysis.

autogpt_platform/backend/backend/copilot/sdk/service.py

@@ -1075,7 +1075,10 @@ async def stream_chat_completion_sdk(
             session_id=session_id,
             trace_name="copilot-sdk",
             tags=["sdk"],
-            metadata={"resume": str(use_resume)},
+            metadata={
+                "resume": str(use_resume),
+                "conversation_turn": turn,
+            },
         )
         _otel_ctx.__enter__()