refactor: separate transit sources and request service

src/layers/transit/source.js

@@ -0,0 +1,22 @@
+import { fetchTransitHistory } from '../../sources/transitHistory.js';
+
+/** Request transit snapshots through caller-owned transport. */
+export function createTransitSource({
+  fetchImpl = (...args) => fetch(...args),
+} = {}) {
+  return {
+    getHistory(feedId, vehicleId, { signal } = {}) {
+      signal?.throwIfAborted();
+      return fetchTransitHistory(feedId, vehicleId, signal, fetchImpl);
+    },
+    requestSnapshot(feedId, { signal } = {}) {
+      signal?.throwIfAborted();
+      if (typeof feedId !== 'string' || !feedId || feedId.length > 160)
+        throw new TypeError('A transit feed identifier is required');
+      return fetchImpl(`/api/transit/vehicles/${encodeURIComponent(feedId)}`, {
+        signal,
+        headers: { Accept: 'application/json' },
+      });
+    },
+  };
+}