refactor: Update API resource URLs to use agency_id variable

Author: Ahmedhossamdev

src/onebusaway/resources/agency.py

@@ -54,7 +54,7 @@ def retrieve(
         if not agency_id:
             raise ValueError(f"Expected a non-empty value for `agency_id` but received {agency_id!r}")
         return self._get(
-            f"/api/where/agency/agencyID.json",
+            f"/api/where/agency/{agency_id}.json",
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -97,7 +97,7 @@ async def retrieve(
         if not agency_id:
             raise ValueError(f"Expected a non-empty value for `agency_id` but received {agency_id!r}")
         return await self._get(
-            f"/api/where/agency/agencyID.json",
+            f"/api/where/agency/{agency_id}.json",
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),