feat(enrollment): implement case of no expiration and already enrolled

this was previously the 409 HTTP status code scenario.
the view test for this case now passes.

benefits/enrollment/views.py

@@ -124,7 +124,7 @@ def index(request):
                 else:
                     if has_no_expiration_date:
                         # no action, return success
-                        pass
+                        return _success(request, group_id)
                     else:
                         # remove expiration date, return success
                         pass
@@ -178,6 +178,11 @@ def _is_within_reenrollment_window(concession_expiry, enrollment_reenrollment_da
     pass
 
 
+def _success(request, group_id):
+    analytics.returned_success(request, group_id)
+    return success(request)
+
+
 @decorator_from_middleware(EligibleSessionRequired)
 def retry(request):
     """View handler for a recoverable failure condition."""