fix: harden multi-file response detection so merge can't fail silently (#6516)

Co-authored-by: Reece Browne <[email protected]>

frontend/editor/src/core/tests/stubbed/merge-response-handling.spec.ts

@@ -0,0 +1,71 @@
+import type { Page, Route } from "@playwright/test";
+import { test, expect } from "@app/tests/helpers/stub-test-base";
+import {
+  uploadFiles,
+  switchToEditorIfViewerMode,
+  runToolAndWaitForReview,
+} from "@app/tests/helpers/ui-helpers";
+import path from "path";
+import fs from "fs";
+
+// Regression coverage: the merge endpoint used to misroute a PDF into JSZip
+// when the Content-Type wasn't an exact `application/pdf`, producing a bogus
+// `result.zip` instead of the merged file. The UI fix uses signature-based
+// detection - %PDF wins regardless of Content-Type.
+
+const FIXTURES_DIR = path.join(__dirname, "../test-fixtures");
+const SAMPLE_PDF = path.join(FIXTURES_DIR, "sample.pdf");
+const SAMPLE_PDF_BYTES = fs.readFileSync(SAMPLE_PDF);
+
+async function mockMergeWithContentType(page: Page, contentType: string) {
+  await page.route("**/api/v1/general/merge-pdfs", (route: Route) =>
+    route.fulfill({
+      status: 200,
+      contentType,
+      headers: {
+        "Content-Disposition": 'attachment; filename="merged_unsigned.pdf"',
+      },
+      body: SAMPLE_PDF_BYTES,
+    }),
+  );
+}
+
+async function runMergeAndOpenReview(page: Page) {
+  await page.goto("/merge");
+  await uploadFiles(page, [SAMPLE_PDF, SAMPLE_PDF]);
+  await switchToEditorIfViewerMode(page);
+  await runToolAndWaitForReview(page);
+}
+
+test.describe("Merge multi-file response detection", () => {
+  test('"application/pdf;charset=UTF-8" -> merged PDF, not result.zip', async ({
+    page,
+  }) => {
+    await mockMergeWithContentType(page, "application/pdf;charset=UTF-8");
+    await runMergeAndOpenReview(page);
+
+    const review = page.locator('[data-testid="review-panel-container"]');
+    await expect(review).toBeVisible();
+    await expect(review.getByText(/result\.zip/i)).toHaveCount(0);
+  });
+
+  test('"application/octet-stream" -> merged PDF, not result.zip', async ({
+    page,
+  }) => {
+    await mockMergeWithContentType(page, "application/octet-stream");
+    await runMergeAndOpenReview(page);
+
+    const review = page.locator('[data-testid="review-panel-container"]');
+    await expect(review).toBeVisible();
+    await expect(review.getByText(/result\.zip/i)).toHaveCount(0);
+  });
+
+  test('"APPLICATION/PDF" -> merged PDF, not result.zip', async ({ page }) => {
+    await mockMergeWithContentType(page, "APPLICATION/PDF");
+    await runMergeAndOpenReview(page);
+
+    const review = page.locator('[data-testid="review-panel-container"]');
+    await expect(review).toBeVisible();
+    await expect(review.getByText(/result\.zip/i)).toHaveCount(0);
+  });
+});