add max length validation for device class strings

Defense-in-depth: validate device class strings don't exceed the
48-byte PROGMEM buffer limit (47 chars + null), matching the same
pattern used for icon strings.

esphome/config_validation.py

@@ -398,6 +398,10 @@ def string_strict(value):
     )
 
 
+# Max device class string length (47 chars + null = 48-byte PROGMEM buffer)
+# Keep in sync with MAX_DEVICE_CLASS_LENGTH in esphome/core/entity_base.h
+DEVICE_CLASS_MAX_LENGTH = 47
+
 # Max icon string length (63 chars + null = 64-byte PROGMEM buffer)
 # Keep in sync with MAX_ICON_LENGTH in esphome/core/entity_base.h
 ICON_MAX_LENGTH = 63

esphome/core/entity_helpers.py

@@ -178,6 +178,10 @@ def _register_string(
 
 def register_device_class(value: str) -> int:
     """Register a device_class string and return its 1-based index."""
+    if value and len(value) > cv.DEVICE_CLASS_MAX_LENGTH:
+        raise ValueError(
+            f"Device class string too long ({len(value)} chars, max {cv.DEVICE_CLASS_MAX_LENGTH}): '{value}'"
+        )
     return _register_string(
         value, _get_pool().device_classes, _MAX_DEVICE_CLASSES, "device_class"
     )

tests/unit_tests/core/test_entity_helpers.py

@@ -23,6 +23,7 @@ from esphome.core.entity_helpers import (
     _setup_entity_impl,
     entity_duplicate_validator,
     get_base_entity_object_id,
+    register_device_class,
     register_icon,
     setup_entity,
 )
@@ -926,6 +927,22 @@ def test_register_icon_max_length() -> None:
     assert register_icon("") == 0
 
 
+def test_register_device_class_max_length() -> None:
+    """Test register_device_class rejects device classes exceeding 47 characters."""
+    # 47 chars should succeed
+    max_dc = "a" * 47
+    idx = register_device_class(max_dc)
+    assert idx > 0
+
+    # 48 chars should fail
+    too_long = "a" * 48
+    with pytest.raises(ValueError, match="Device class string too long"):
+        register_device_class(too_long)
+
+    # Empty string returns 0
+    assert register_device_class("") == 0
+
+
 @pytest.mark.asyncio
 async def test_setup_entity_with_entity_category(
     setup_test_environment: list[str],