MM-46874: Change to 400 for failed to upgrade ws (#21492)

https://mattermost.atlassian.net/browse/MM-46874

```release-note
NONE
```

api4/apitestlib.go

@@ -71,6 +71,7 @@ type TestHelper struct {
 
 	IncludeCacheLayer bool
 
+	LogBuffer  *mlog.Buffer
 	TestLogger *mlog.Logger
 }
 
@@ -119,11 +120,16 @@ func setupTestHelper(dbStore store.Store, searchEngine *searchengine.Broker, ent
 		options = append(options, app.StoreOverride(dbStore))
 	}
 
+	buffer := &mlog.Buffer{}
+
 	testLogger, _ := mlog.NewLogger()
 	logCfg, _ := config.MloggerConfigFromLoggerConfig(&memoryConfig.LogSettings, nil, config.GetLogFileLocation)
 	if errCfg := testLogger.ConfigureTargets(logCfg, nil); errCfg != nil {
 		panic("failed to configure test logger: " + errCfg.Error())
 	}
+	if errW := mlog.AddWriterTarget(testLogger, buffer, true, mlog.StdAll...); errW != nil {
+		panic("failed to add writer target to test logger: " + errW.Error())
+	}
 	// lock logger config so server init cannot override it during testing.
 	testLogger.LockConfiguration()
 	options = append(options, app.SetLogger(testLogger))
@@ -140,6 +146,7 @@ func setupTestHelper(dbStore store.Store, searchEngine *searchengine.Broker, ent
 		IncludeCacheLayer: includeCache,
 		Context:           request.EmptyContext(testLogger),
 		TestLogger:        testLogger,
+		LogBuffer:         buffer,
 	}
 	th.Context.SetLogger(testLogger)
 

api4/websocket.go

@@ -32,7 +32,7 @@ func connectWebSocket(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	ws, err := upgrader.Upgrade(w, r, nil)
 	if err != nil {
-		c.Err = model.NewAppError("connect", "api.web_socket.connect.upgrade.app_error", nil, err.Error(), http.StatusInternalServerError)
+		c.Err = model.NewAppError("connect", "api.web_socket.connect.upgrade.app_error", nil, err.Error(), http.StatusBadRequest)
 		return
 	}
 

api4/websocket_test.go

@@ -15,6 +15,8 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/testlib"
 )
 
 func TestWebSocketTrailingSlash(t *testing.T) {
@@ -417,3 +419,15 @@ func TestWebSocketStatuses(t *testing.T) {
 
 	WebSocketClient.Close()
 }
+
+func TestWebSocketUpgrade(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	url := fmt.Sprintf("http://localhost:%v", th.App.Srv().ListenAddr.Port) + model.APIURLSuffix + "/websocket"
+	resp, err := http.Get(url)
+	require.NoError(t, err)
+	require.Equal(t, resp.StatusCode, http.StatusBadRequest)
+	require.NoError(t, th.TestLogger.Flush())
+	testlib.AssertLog(t, th.LogBuffer, mlog.LvlDebug.Name, "Failed to upgrade websocket connection.")
+}