test(chat): make RateLimiter mock a constructable class

The arrow-function mockImplementation form was not reliably constructable
in the full suite run (`new RateLimiter()` threw "is not a constructor"),
though it passed in isolation. Switch to the class-based mock used by the
sibling OTP/speech route tests.

Author: waleedlatif1

apps/sim/app/api/chat/utils.test.ts

@@ -31,9 +31,9 @@ const {
 }))
 
 vi.mock('@/lib/core/rate-limiter', () => ({
-  RateLimiter: vi.fn().mockImplementation(() => ({
-    checkRateLimitDirect: mockCheckRateLimitDirect,
-  })),
+  RateLimiter: class {
+    checkRateLimitDirect = mockCheckRateLimitDirect
+  },
 }))
 
 vi.mock('@/lib/auth', () => ({