protondrive: follow the renamed Proton code constant in the fake's tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/protondrive/protontest/server_test.go

@@ -150,7 +150,7 @@ func TestFaultInjection(t *testing.T) {
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 		resp, body := ping(t, s)
 		assert.Equal(t, http.StatusServiceUnavailable, resp.StatusCode)
-		assert.Equal(t, api.CodeAppVersionBad, decodeError(t, body).Code)
+		assert.Equal(t, api.CodeFeatureBlocked, decodeError(t, body).Code)
 		resp, _ = ping(t, s)
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 	})