fix(backend/copilot): handle blank lines in validate_transcript

Skip empty lines instead of treating them as parse errors,
preventing silent data loss from transcripts with blank lines.

Addresses PR comment #2894841670

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

@@ -223,6 +223,8 @@ def validate_transcript(content: str | None) -> bool:
     has_assistant = False
 
     for line in lines:
+        if not line.strip():
+            continue
         try:
             entry = json.loads(line)
             if entry.get("type") == "assistant":