feat: add /api/v1/language endpoint for Go/Python runtime detection (#16952)

Both backends serve GET /api/v1/language. Frontend calls it once and
caches. By this way, front end can know the backend is go or python and
thus can determine which part of logic to load.

---------

Co-authored-by: Claude <noreply@anthropic.com>

web/src/utils/api.ts

@@ -181,6 +181,7 @@ export default {
     `${restAPIv1}/datasets/${datasetId}/ingestions/${logId}`,
   fetchPipelineDatasetLogs: (datasetId: string) =>
     `${restAPIv1}/datasets/${datasetId}/ingestions`,
+  listPipelines: `${restAPIv1}/pipelines?type=builtin`,
   runIndex: (datasetId: string, indexType: string) =>
     `${restAPIv1}/datasets/${datasetId}/index?type=${indexType.toLowerCase()}`,
   traceIndex: (datasetId: string, indexType: string) =>

web/src/utils/backend-runtime.ts

@@ -0,0 +1,40 @@
+/**
+ * Backend runtime language detection.
+ *
+ * Fetches /api/v1/language once at module load (app start) and caches the
+ * result. Components subscribe via React's useSyncExternalStore; there is no
+ * per-component fetch or useEffect.
+ *
+ * Pattern: module-level fetch + listener set, same subscription approach as
+ * enterprise billingStatus.ts.
+ */
+
+type Listener = () => void;
+const listeners = new Set<Listener>();
+
+let backendLanguage: string | null = null;
+
+// Kick off the fetch at module load — app start, not component mount.
+const promise: Promise<string> = fetch('/api/v1/language')
+  .then((r) => r.json())
+  .then((body: { data?: { language?: string } }) => {
+    backendLanguage = body.data?.language === 'go' ? 'go' : 'python';
+    listeners.forEach((fn) => fn());
+    return backendLanguage;
+  })
+  .catch(() => {
+    backendLanguage = 'python';
+    listeners.forEach((fn) => fn());
+    return 'python';
+  });
+
+export const fetchBackendLanguage = (): Promise<string> => promise;
+
+export const getBackendLanguage = (): string | null => backendLanguage;
+
+export const subscribeBackendLanguage = (listener: Listener): (() => void) => {
+  listeners.add(listener);
+  return () => {
+    listeners.delete(listener);
+  };
+};