fix(backend/copilot): don't short-circuit JSONL validation in validate_transcript

Remove `break` after finding first assistant entry so all remaining
lines are still validated for JSON correctness. Without this, corrupted
JSONL after the first assistant entry would slip through and get
uploaded as a broken --resume file.

autogpt_platform/backend/backend/copilot/sdk/transcript.py

@@ -268,7 +268,6 @@ def validate_transcript(content: str | None) -> bool:
             entry = json.loads(line)
             if entry.get("type") == "assistant":
                 has_assistant = True
-                break
         except json.JSONDecodeError:
             return False