debug: add detailed error reporting for session detail serialization

Temporary debug commit — surfaces the actual exception when GET /ai-sessions/{id}
fails with 500, instead of generic "Internal server error" from middleware.

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

backend/app/api/endpoints/ai_sessions.py

@@ -645,9 +645,16 @@ async def get_session(
             confidence_score=step.confidence_at_step,
         ))
 
-    detail = AISessionDetail.model_validate(session)
-    detail.steps = step_responses
-    return detail
+    try:
+        detail = AISessionDetail.model_validate(session)
+        detail.steps = step_responses
+        return detail
+    except Exception as e:
+        logger.exception("Failed to serialize session %s: %s", session_id, e)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Session serialization error: {type(e).__name__}: {str(e)[:200]}",
+        )
 
 
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