chore: Add global fetch mock in vitest.setup.ts to suppress happy-dom ECONNREFUSED errors (#35131)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

web/vitest.setup.ts

@@ -74,6 +74,18 @@ if (typeof globalThis.IntersectionObserver === 'undefined') {
   }
 }
 
+// Mock global fetch to prevent happy-dom from making real network calls
+// (which would cause ECONNREFUSED errors against localhost:5001).
+// Individual tests can still override via vi.spyOn(globalThis, 'fetch') or reassignment.
+globalThis.fetch = vi.fn(() =>
+  Promise.resolve(
+    new Response(JSON.stringify({}), {
+      status: 200,
+      headers: { 'Content-Type': 'application/json' },
+    }),
+  ),
+) as unknown as typeof fetch
+
 afterEach(async () => {
   // Wrap cleanup in act() to flush pending React scheduler work
   // This prevents "window is not defined" errors from React 19's scheduler