fix(copilot): prevent duplicate error markers and extract shared helper

- Change stale error marker cleanup from `if` to `while` so ALL
  trailing markers are removed (fixes issue where mid-stream error
  + cleanup error could leave a stale marker)
- Skip appending error marker in BaseException handler when one was
  already appended inside the stream loop (ended_with_stream_error)
- Extract `_append_error_marker()` helper to deduplicate marker
  appending logic across 4 call sites
- Update misleading "mark as retryable" comment to match actual
  behavior (uses retryable prefix, not a model field)
- Add docstring to `_safe()` helper
- Remove unused `prefix` variable from stream error tuple

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

@@ -660,7 +660,10 @@ async def stream_chat_completion_sdk(
         len(session.messages) > 0
         and session.messages[-1].role == "assistant"
         and session.messages[-1].content
-        and COPILOT_ERROR_PREFIX in session.messages[-1].content
+        and (
+            COPILOT_ERROR_PREFIX in session.messages[-1].content
+            or COPILOT_RETRYABLE_ERROR_PREFIX in session.messages[-1].content
+        )
     ):
         logger.info(
             "[SDK] [%s] Removing stale error marker from previous turn",