Fix duplicate polygon queries causing redundant API calls

Bug: When the Govmap API returns the same polygon_id multiple times
(which can happen for addresses with multiple associated polygons),
the code was processing each duplicate separately, leading to:
- Duplicate API calls for street/neighborhood deals
- Wasted API quota and slower performance
- Confusing logs showing same polygon queried multiple times

Example from logs:
  INFO: Querying polygon 53283601 (distance: 0m from search point)
  INFO: Getting street deals for polygon: 53283601
  INFO: Querying polygon 53283601 (distance: 0m from search point)
  INFO: Getting street deals for polygon: 53283601

Root cause:
- nearby_polygons API response can contain duplicate polygon_ids
- Code was appending all polygons without deduplication
- Log message claimed "unique polygon IDs" but didn't enforce it

Solution:
- Use dict to deduplicate polygons by polygon_id (lines 612-643)
- When duplicates exist, keep the one with shortest distance
- Convert dict to list after deduplication
- Now truly have unique polygon IDs as the log claims

Impact:
- Reduces API calls (fewer duplicates = less load on Govmap API)
- Faster queries (less redundant processing)
- Clearer logs (no confusing duplicate entries)
- Better performance for addresses with many associated polygons

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nadlan_mcp/govmap/client.py

@@ -608,12 +608,15 @@ class GovmapClient:
             logger.info(f"Using coordinates: {point}")
 
             # Extract polygon metadata with coordinates for distance calculation
-            polygon_metadata_list = []
+            # Use dict to deduplicate by polygon_id (keep closest occurrence)
+            polygon_dict = {}
             for metadata in nearby_polygons:
                 polygon_id = metadata.get("polygon_id")
                 if not polygon_id:
                     continue
 
+                polygon_id_str = str(polygon_id)
+
                 # Extract polygon center coordinates (use shape if available, fallback to search point)
                 # Most polygon metadata includes shape which can be parsed for center
                 # For now, use metadata coordinates if available, otherwise estimate
@@ -628,9 +631,19 @@ class GovmapClient:
                 if poly_coords:
                     distance = utils.calculate_distance(point, poly_coords)
 
-                polygon_metadata_list.append(
-                    {"polygon_id": str(polygon_id), "distance": distance, "metadata": metadata}
-                )
+                # Deduplicate: keep the polygon with shortest distance if duplicate IDs exist
+                if (
+                    polygon_id_str not in polygon_dict
+                    or distance < polygon_dict[polygon_id_str]["distance"]
+                ):
+                    polygon_dict[polygon_id_str] = {
+                        "polygon_id": polygon_id_str,
+                        "distance": distance,
+                        "metadata": metadata,
+                    }
+
+            # Convert dict to list
+            polygon_metadata_list = list(polygon_dict.values())
 
             # Sort polygons by distance (closest first)
             polygon_metadata_list.sort(key=lambda x: x["distance"])