fix: keep transit snapshot parsing within request lifetime

src/layers/transit/source.js

@@ -13,9 +13,23 @@ export function createTransitSource({
       signal?.throwIfAborted();
       if (typeof feedId !== 'string' || !feedId || feedId.length > 160)
         throw new TypeError('A transit feed identifier is required');
-      return fetchImpl(`/api/transit/vehicles/${encodeURIComponent(feedId)}`, {
-        signal,
-        headers: { Accept: 'application/json' },
+      return Promise.resolve(
+        fetchImpl(`/api/transit/vehicles/${encodeURIComponent(feedId)}`, {
+          signal,
+          headers: { Accept: 'application/json' },
+        }),
+      ).then((response) => {
+        signal?.throwIfAborted();
+        return {
+          ok: response.ok,
+          status: response.status,
+          headers: response.headers,
+          async json() {
+            const body = await response.json();
+            signal?.throwIfAborted();
+            return body;
+          },
+        };
       });
     },
   };

src/tooling/transitService.test.mjs

@@ -110,3 +110,32 @@ test('source retains history validation and never retries through another transp
   );
   assert.equal(calls, 1);
 });
+
+test('snapshot cancellation covers headers and body parsing', async () => {
+  for (const phase of ['headers', 'body']) {
+    const controller = new AbortController();
+    const source = createTransitSource({
+      fetchImpl: async () => {
+        if (phase === 'headers') controller.abort();
+        return {
+          ok: true,
+          status: 200,
+          headers: new Headers(),
+          json: async () => {
+            controller.abort();
+            return { vehicles: [] };
+          },
+        };
+      },
+    });
+    await assert.rejects(
+      async () => {
+        const response = await source.requestSnapshot('mbta', {
+          signal: controller.signal,
+        });
+        await response.json();
+      },
+      { name: 'AbortError' },
+    );
+  }
+});