fix: add raw_output to signal processing fallback

Preserves the LLM's natural language output in the fallback JSON
when JSON parsing fails, for debugging and manual review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tradingagents/prediction_market/graph/signal_processing.py

@@ -68,4 +68,5 @@ Respond with ONLY valid JSON, no other text. Example:
                 "edge": None,
                 "position_size": None,
                 "confidence": None,
+                "raw_output": result,
             })