[api] Add max_data_length = 63 for unit_of_measurement fields

Add UNIT_OF_MEASUREMENT_MAX_LENGTH = 63 constant in core/config.py
and enforce it in sensor and number component validators.

Annotate unit_of_measurement proto fields with (max_data_length) = 63
so the codegen emits constant-size length varint calculations.

esphome/components/api/api.proto

@@ -312,6 +312,7 @@ enum EntityCategory {
 //   name/object_id = 120 (NAME_MAX_LENGTH)
 //   icon = 63 (ICON_MAX_LENGTH)
 //   device_class = 47 (DEVICE_CLASS_MAX_LENGTH)
+//   unit_of_measurement = 63 (UNIT_OF_MEASUREMENT_MAX_LENGTH)
 
 // ==================== BINARY SENSOR ====================
 message ListEntitiesBinarySensorResponse {
@@ -637,7 +638,7 @@ message ListEntitiesSensorResponse {
   reserved 4; // Deprecated: was string unique_id
 
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
-  string unit_of_measurement = 6;
+  string unit_of_measurement = 6 [(max_data_length) = 63];
   int32 accuracy_decimals = 7;
   bool force_update = 8;
   string device_class = 9 [(max_data_length) = 47];

esphome/components/api/api_pb2.cpp

@@ -707,7 +707,7 @@ uint32_t ListEntitiesSensorResponse::calculate_size() const {
 #ifdef USE_ENTITY_ICON
   size += !this->icon.empty() ? 2 + this->icon.size() : 0;
 #endif
-  size += ProtoSize::calc_length(1, this->unit_of_measurement.size());
+  size += !this->unit_of_measurement.empty() ? 2 + this->unit_of_measurement.size() : 0;
   size += ProtoSize::calc_int32(1, this->accuracy_decimals);
   size += ProtoSize::calc_bool(1, this->force_update);
   size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;

esphome/components/number/__init__.py

@@ -79,6 +79,7 @@ from esphome.const import (
     DEVICE_CLASS_WIND_SPEED,
 )
 from esphome.core import CORE, CoroPriority, coroutine_with_priority
+from esphome.core.config import UNIT_OF_MEASUREMENT_MAX_LENGTH
 from esphome.core.entity_helpers import (
     entity_duplicate_validator,
     setup_device_class,
@@ -186,7 +187,11 @@ NUMBER_OPERATION_OPTIONS = {
 }
 
 validate_device_class = cv.one_of(*DEVICE_CLASSES, lower=True, space="_")
-validate_unit_of_measurement = cv.string_strict
+validate_unit_of_measurement = cv.All(
+    cv.string_strict,
+    # Keep in sync with max_data_length in api.proto
+    cv.Length(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
+)
 
 _NUMBER_SCHEMA = (
     cv.ENTITY_BASE_SCHEMA.extend(web_server.WEBSERVER_SORTING_SCHEMA)

esphome/components/sensor/__init__.py

@@ -106,6 +106,7 @@ from esphome.const import (
     ENTITY_CATEGORY_CONFIG,
 )
 from esphome.core import CORE, CoroPriority, coroutine_with_priority
+from esphome.core.config import UNIT_OF_MEASUREMENT_MAX_LENGTH
 from esphome.core.entity_helpers import (
     entity_duplicate_validator,
     setup_device_class,
@@ -290,7 +291,11 @@ ClampFilter = sensor_ns.class_("ClampFilter", Filter)
 RoundFilter = sensor_ns.class_("RoundFilter", Filter)
 RoundMultipleFilter = sensor_ns.class_("RoundMultipleFilter", Filter)
 
-validate_unit_of_measurement = cv.string_strict
+validate_unit_of_measurement = cv.All(
+    cv.string_strict,
+    # Keep in sync with max_data_length in api.proto
+    cv.Length(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
+)
 validate_accuracy_decimals = cv.int_
 validate_icon = cv.icon
 validate_device_class = cv.one_of(*DEVICE_CLASSES, lower=True, space="_")

esphome/core/config.py

@@ -233,6 +233,9 @@ DEVICE_CLASS_MAX_LENGTH = 47
 # Keep in sync with MAX_ICON_LENGTH in esphome/core/entity_base.h
 ICON_MAX_LENGTH = 63
 
+# Max unit of measurement string length
+UNIT_OF_MEASUREMENT_MAX_LENGTH = 63
+
 AREA_SCHEMA = cv.Schema(
     {
         cv.GenerateID(CONF_ID): cv.declare_id(Area),