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;
+          },
+        };
       });
     },
   };