Short-circuit API rate limiter for unauthenticated user

Calls by unauthenticated users were failing at API rate limiter as it
failed to access user info object. This is a bug.

API rate limiter should short-circuit for unauthenicated users so a
proper Forbidden response can be returned by API

Add regression test to verify that unauthenticated users get 403
response when calling the /chat API endpoint

src/khoj/routers/helpers.py

@@ -299,6 +299,11 @@ class ApiUserRateLimiter:
         self.cache: dict[str, list[float]] = defaultdict(list)
 
     def __call__(self, request: Request):
+        # Rate limiting is disabled if user unauthenticated.
+        # Other systems handle authentication
+        if not request.user.is_authenticated:
+            return
+
         user: KhojUser = request.user.object
         subscribed = has_required_scope(request, ["premium"])
         user_requests = self.cache[user.uuid]