[api] Add max_data_length proto option and optimize entity string encoding

Add a max_data_length field option to api_options.proto for string/bytes
fields. When max_data_length < 128 and force = true, the code generator
uses encode_short_string_force() — a single call that writes the tag
byte, 1-byte length varint, and raw string data with no branching. Size
calculation simplifies from calc_length(1, size) to 2 + size.

Annotate entity fields across all 25 ListEntities*Response messages:
- name and object_id: max_data_length = 120, force = true (50 fields)
- icon: max_data_length = 63 (25 fields)

The 120 and 63 values match NAME_MAX_LENGTH and ICON_MAX_LENGTH in
esphome/core/config.py, validated at config time.

tests/unit_tests/core/test_entity_helpers.py

@@ -28,6 +28,7 @@ from esphome.core.entity_helpers import (
     get_base_entity_object_id,
     register_device_class,
     register_icon,
+    register_unit_of_measurement,
     setup_device_class,
     setup_entity,
     setup_unit_of_measurement,
@@ -925,6 +926,22 @@ def test_register_device_class_max_length() -> None:
     assert register_device_class("") == 0
 
 
+def test_register_unit_of_measurement_max_length() -> None:
+    """Test register_unit_of_measurement rejects units exceeding 63 characters."""
+    # 63 chars should succeed
+    max_uom = "a" * 63
+    idx = register_unit_of_measurement(max_uom)
+    assert idx > 0
+
+    # 64 chars should fail
+    too_long = "a" * 64
+    with pytest.raises(ValueError, match="Unit of measurement string too long"):
+        register_unit_of_measurement(too_long)
+
+    # Empty string returns 0
+    assert register_unit_of_measurement("") == 0
+
+
 @pytest.mark.asyncio
 async def test_setup_entity_with_entity_category(
     setup_test_environment: list[str],