fix(test): make FormData content-type assertion type-safe

src/test/api/api-formdata-content-type.test.ts

@@ -49,10 +49,12 @@ describe("api.post FormData Content-Type", () => {
 
     expect(capturedContentType).not.toBe("application/json");
     // Browser sets multipart/form-data; boundary=... when Content-Type was stripped.
-    // In jsdom/fetch mock the runtime may leave it null or set multipart — both are OK
+    // In jsdom/fetch mock the runtime may leave it null or set multipart - both are OK
     // as long as it is not forced JSON.
-    if (capturedContentType) {
-      expect(capturedContentType.toLowerCase()).toContain("multipart/form-data");
+    // Cast after expect(): Vitest assertion narrowing collapses string|null to never.
+    const contentType = capturedContentType as string | null;
+    if (contentType !== null) {
+      expect(contentType.toLowerCase()).toContain("multipart/form-data");
     }
   });
 });