Fix missing color_mode initialization in MQTT JSON light schema

Initialize `_fixed_color_mode` and `_attr_color_mode` before the
color mode branching logic in `_setup_from_config`, matching the
pattern already used in `schema_template.py` (added in #162715).

Without this, `_attr_color_mode` can remain `None` (the new default
from #162715) in edge cases during entity (re)setup, causing
"does not report a color mode" errors when the light is on.

Also use `_fixed_color_mode` in `_update_color()` to skip color
mode changes for single-mode lights, preventing the fixed mode
from being overwritten by state updates. This matches the guard
pattern used in `schema_template.py`.

Author: noerstad

homeassistant/components/mqtt/light/schema_json.py

@@ -190,10 +190,13 @@ def _setup_from_config(self, config: ConfigType) -> None:
         self._attr_supported_features |= (
             config[CONF_TRANSITION] and LightEntityFeature.TRANSITION
         )
+        self._fixed_color_mode = None
+        self._attr_color_mode = ColorMode.UNKNOWN
         if supported_color_modes := self._config.get(CONF_SUPPORTED_COLOR_MODES):
             self._attr_supported_color_modes = supported_color_modes
             if self.supported_color_modes and len(self.supported_color_modes) == 1:
-                self._attr_color_mode = next(iter(self.supported_color_modes))
+                self._fixed_color_mode = next(iter(self.supported_color_modes))
+                self._attr_color_mode = self._fixed_color_mode
             else:
                 self._attr_color_mode = ColorMode.UNKNOWN
         elif config.get(CONF_BRIGHTNESS):
@@ -206,6 +209,8 @@ def _setup_from_config(self, config: ConfigType) -> None:
             self._attr_color_mode = ColorMode.ONOFF
 
     def _update_color(self, values: dict[str, Any]) -> None:
+        if self._fixed_color_mode:
+            return
         color_mode: str = values["color_mode"]
         if not self._supports_color_mode(color_mode):
             _LOGGER.warning(