fix(chat): stop 429 retry spiral and add get_weather coord fallback

- search_places: detect HTTP 429 and mark retryable=False to stop the
  retry loop immediately instead of spiraling until MAX_ITERATIONS
- get_weather: extract collection coordinates (lat/lng from first
  location with coords) and retry when LLM omits required params;
  uses sync_to_async for the DB query in the async view
- AITravelChat: deduplicate context-only tools (get_trip_details,
  get_weather) in the render pipeline to prevent duplicate place cards
  from appearing when the retry loop causes multiple get_trip_details calls
- Tests: 5 new tests covering 429 non-retryable path and weather
  coord fallback; all 39 chat tests pass

backend/server/chat/agent_tools.py

@@ -196,6 +196,10 @@ def search_places(
             "category": category,
             "results": results,
         }
+    except requests.HTTPError as exc:
+        if exc.response is not None and exc.response.status_code == 429:
+            return {"error": f"Places API request failed: {exc}", "retryable": False}
+        return {"error": f"Places API request failed: {exc}"}
     except requests.RequestException as exc:
         return {"error": f"Places API request failed: {exc}"}
     except (TypeError, ValueError) as exc: