Add support for constant selector (#89573)

* Add support for constant selector

* Adapt to frontend PR changes

homeassistant/helpers/selector.py

@@ -358,6 +358,38 @@ class ConfigEntrySelector(Selector[ConfigEntrySelectorConfig]):
         return config
 
 
+class ConstantSelectorConfig(TypedDict, total=False):
+    """Class to represent a constant selector config."""
+
+    label: str
+    translation_key: str
+    value: str | int | bool
+
+
+@SELECTORS.register("constant")
+class ConstantSelector(Selector[ConstantSelectorConfig]):
+    """Constant selector."""
+
+    selector_type = "constant"
+
+    CONFIG_SCHEMA = vol.Schema(
+        {
+            vol.Optional("label"): str,
+            vol.Optional("translation_key"): cv.string,
+            vol.Required("value"): vol.Any(str, int, bool),
+        }
+    )
+
+    def __init__(self, config: ConstantSelectorConfig | None = None) -> None:
+        """Instantiate a selector."""
+        super().__init__(config)
+
+    def __call__(self, data: Any) -> Any:
+        """Validate the passed selection."""
+        vol.Schema(self.config["value"])(data)
+        return self.config["value"]
+
+
 class DateSelectorConfig(TypedDict):
     """Class to represent a date selector config."""