fix: use correct Grist API endpoint for modify_column

The Grist API uses PATCH /tables/{table}/columns with a columns array
in the body, not PATCH /tables/{table}/columns/{column_id}. Updated
the endpoint to match the API spec.

src/grist_mcp/grist_client.py

@@ -160,7 +160,8 @@ class GristClient:
         if formula is not None:
             fields["formula"] = formula
 
-        await self._request("PATCH", f"/tables/{table}/columns/{column_id}", json={"fields": fields})
+        payload = {"columns": [{"id": column_id, "fields": fields}]}
+        await self._request("PATCH", f"/tables/{table}/columns", json=payload)
 
     async def delete_column(self, table: str, column_id: str) -> None:
         """Delete a column from a table."""

tests/unit/test_grist_client.py

@@ -160,7 +160,7 @@ async def test_add_column(client, httpx_mock: HTTPXMock):
 @pytest.mark.asyncio
 async def test_modify_column(client, httpx_mock: HTTPXMock):
     httpx_mock.add_response(
-        url="https://grist.example.com/api/docs/abc123/tables/Table1/columns/Amount",
+        url="https://grist.example.com/api/docs/abc123/tables/Table1/columns",
         method="PATCH",
         json={},
     )