feat: read the transit feed catalog through the transit source

Add getFeeds() to the transit source. It reads the public catalog route
the server already serves, so code outside the layer can find feeds
near a location without loading the layer.

scripts/format-scope.json

@@ -873,5 +873,6 @@
   "src/tools/queries/aviation.test.mjs",
   "src/tools/queries/space.test.mjs",
   "src/tools/queries/media.test.mjs",
-  "src/tools/queries/places.test.mjs"
+  "src/tools/queries/places.test.mjs",
+  "src/layers/transit/source.test.mjs"
 ]

src/layers/transit/source.js

@@ -5,6 +5,21 @@ export function createTransitSource({
   fetchImpl = (...args) => fetch(...args),
 } = {}) {
   return {
+    /** Read the public catalog of enabled feeds. */
+    async getFeeds({ signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl('/api/transit/feeds', {
+        signal,
+        headers: { Accept: 'application/json' },
+      });
+      if (!response.ok)
+        throw new Error('Transit feeds HTTP ' + response.status);
+      const body = await response.json();
+      signal?.throwIfAborted();
+      if (!Array.isArray(body?.feeds))
+        throw new Error('Malformed transit feed catalog');
+      return body.feeds;
+    },
     getHistory(feedId, vehicleId, { signal } = {}) {
       signal?.throwIfAborted();
       return fetchTransitHistory(feedId, vehicleId, signal, fetchImpl);

src/layers/transit/source.test.mjs

@@ -0,0 +1,46 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { createTransitSource } from './source.js';
+
+const json = (body, status = 200) =>
+  new Response(JSON.stringify(body), {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+
+test('the feed catalog is read from the public catalog route', async () => {
+  const requested = [];
+  const feeds = [
+    {
+      id: 'mbta',
+      name: 'MBTA',
+      center: { lat: 42.36, lon: -71.06 },
+      loadRadiusKm: 60,
+    },
+  ];
+  const source = createTransitSource({
+    fetchImpl: async (url) => {
+      requested.push(url);
+      return json({ feeds });
+    },
+  });
+  assert.deepEqual(await source.getFeeds(), feeds);
+  assert.deepEqual(requested, ['/api/transit/feeds']);
+});
+
+test('feed catalog failures are reported', async () => {
+  const failing = createTransitSource({ fetchImpl: async () => json({}, 503) });
+  await assert.rejects(failing.getFeeds(), /Transit feeds HTTP 503/);
+  const malformed = createTransitSource({
+    fetchImpl: async () => json({ feeds: {} }),
+  });
+  await assert.rejects(malformed.getFeeds(), /Malformed transit feed catalog/);
+  const aborted = new AbortController();
+  aborted.abort();
+  await assert.rejects(
+    createTransitSource({
+      fetchImpl: async () => json({ feeds: [] }),
+    }).getFeeds({ signal: aborted.signal }),
+    (error) => error.name === 'AbortError',
+  );
+});