[GH-19150] Remove strings in all channel types for constants (#19535)

Automatic Merge

model/channel_test.go

@@ -62,7 +62,7 @@ func TestChannelIsValid(t *testing.T) {
 	o.Type = "U"
 	require.NotNil(t, o.IsValid())
 
-	o.Type = "P"
+	o.Type = ChannelTypePrivate
 	require.Nil(t, o.IsValid())
 
 	o.Header = strings.Repeat("01234567890", 100)