test(e2e): add seedFlowViaApi fixture to seed flows via the API

Add a seedFlowViaApi Playwright fixture in tests/fixtures.ts that creates a flow through POST /api/v1/flows/ instead of rebuilding it on the canvas. It reuses the page's authenticated same-origin session and returns the created flow, so specs can navigate straight to /flow/<id> without driving the UI to build state.

Foundation for Phase 0 (F1): the biggest unlock for removing UI-rebuild cost across the unit and feature suites. No specs consume it yet; first consumers land with P1/P3.

Refs LE-1665

src/frontend/tests/fixtures.ts

@@ -15,6 +15,27 @@ import type { A11yScanOptions, LangflowPage } from "./utils/types";
 
 export type { A11yScanOptions, LangflowPage } from "./utils/types";
 
+/**
+ * Options for the `seedFlowViaApi` fixture. `data` is the flow graph
+ * (`{ nodes, edges, viewport }`); omit it to seed an empty flow.
+ */
+export type SeedFlowOptions = {
+  name?: string;
+  description?: string;
+  data?: Record<string, unknown>;
+  folderId?: string;
+};
+
+/** Subset of the FlowRead payload returned by `POST /api/v1/flows/`. */
+export type SeededFlow = {
+  id: string;
+  name: string;
+  [key: string]: unknown;
+};
+
+/** Seeds a flow through the API and resolves to the created flow. */
+export type SeedFlowViaApi = (options?: SeedFlowOptions) => Promise<SeededFlow>;
+
 const RUN_A11Y = process.env.RUN_A11Y === "true";
 const RUN_A11Y_ASSERT = process.env.RUN_A11Y_ASSERT === "true";
 
@@ -32,7 +53,10 @@ const CPU_THROTTLE_RATE = (() => {
 })();
 
 // Extend test to log backend errors
-export const test = base.extend<{ page: LangflowPage }, A11yFixtures>({
+export const test = base.extend<
+  { page: LangflowPage; seedFlowViaApi: SeedFlowViaApi },
+  A11yFixtures
+>({
   _a11ySession: [
     async ({ browserName }, use) => {
       void browserName;
@@ -339,6 +363,44 @@ export const test = base.extend<{ page: LangflowPage }, A11yFixtures>({
       }
     }
   },
+
+  /**
+   * Seed a flow straight through the API instead of rebuilding it on the
+   * canvas. Uses the page's authenticated same-origin session, so call it
+   * after the test has bootstrapped/logged in. Returns the created flow —
+   * navigate with `page.goto(`/flow/${flow.id}`)`.
+   */
+  seedFlowViaApi: async ({ page }, use) => {
+    await use(async (options: SeedFlowOptions = {}) => {
+      const payload = {
+        name: options.name ?? `seeded-flow-${Date.now()}`,
+        description: options.description ?? "Seeded via API for e2e",
+        data: options.data ?? {
+          nodes: [],
+          edges: [],
+          viewport: { x: 0, y: 0, zoom: 1 },
+        },
+        ...(options.folderId ? { folder_id: options.folderId } : {}),
+      };
+
+      const created = await page.evaluate(async (body) => {
+        const response = await fetch("/api/v1/flows/", {
+          method: "POST",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify(body),
+        });
+        if (!response.ok) {
+          const detail = await response.text();
+          throw new Error(
+            `seedFlowViaApi: POST /api/v1/flows/ failed (${response.status}): ${detail}`,
+          );
+        }
+        return response.json();
+      }, payload);
+
+      return created as SeededFlow;
+    });
+  },
 });
 
 export { expect };