GH-27059: Rewrite error messages about msg length (#29252)

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/public/model/post_test.go

@@ -59,6 +59,11 @@ func TestPostIsValid(t *testing.T) {
 	appErr = o.IsValid(maxPostSize)
 	require.NotNil(t, appErr)
 
+	// In case message property length is too long.
+	o.Message = strings.Repeat("0", maxPostSize+1)
+	appErr = o.IsValid(maxPostSize)
+	require.NotNil(t, appErr)
+
 	o.Message = strings.Repeat("0", maxPostSize)
 	appErr = o.IsValid(maxPostSize)
 	require.Nil(t, appErr)