feat: add buffer tool (#8)

* wip buffer tool

* add search area to attribute

* add geopandas dependency

* add buffer tool and fix graph to add in the overture tool

* make async

* simpler buffering

* cleaner comments

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

* Change from FeatureCollection to Feature

And ensure that output search area is a Polygon

* update test comments

* remove unhelpful test checks

---------

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>
Co-authored-by: Daniel Wiesmann <yellowcap@users.noreply.github.com>

tests/tools/test_buffer.py

@@ -0,0 +1,40 @@
+from pytest import fixture
+from geo_assistant.agent.state import GeoAssistantState
+from geo_assistant.tools.buffer import get_search_area
+from geojson_pydantic import Feature, Point
+from langchain_core.tools.base import ToolCall
+
+
+@fixture
+def geo_assistant_fixture():
+    place_geojson = Feature(
+        type="Feature",
+        geometry=Point(type="Point", coordinates=[-9.1393, 38.7223]),
+        properties={"name": "Neighbourhood Cafe Lisbon"},
+    )
+    return GeoAssistantState(place=place_geojson, search_area=None, messages=[])
+
+
+async def test_get_search_area(geo_assistant_fixture):
+    # Call the underlying function directly to test the logic
+    # This bypasses the injection framework which is better suited for integration tests
+    command = await get_search_area.ainvoke(
+        ToolCall(
+            name="get_search_area",
+            type="tool_call",
+            id="test_id_search_area",
+            args={
+                "buffer_size_km": 10.0,
+                "state": geo_assistant_fixture,
+                "tool_call_id": "test_id_search_area",
+            },
+        ),
+    )
+
+    # Verify the state was used correctly
+    assert "search_area" in command.update
+    assert "messages" in command.update
+
+    # Verify the buffer was created around the correct place
+    search_area = command.update["search_area"]
+    assert search_area["type"] == "Polygon"