[api] Auto-derive max_value for enum fields in protobuf codegen

The protobuf code generator already parses all enum definitions and
knows every enum's maximum value. Use this to automatically apply the
max_value < 128 optimization to all enum fields, eliminating the need
for manual annotation.

Since every enum in api.proto has max < 128, all 58 enum field size
calculations now use constant arithmetic instead of calling varint
size functions.

Builds on #15424 which introduced the max_value optimization.

script/api_protobuf/api_protobuf.py

@@ -56,6 +56,10 @@ FILE_HEADER = """// This file was automatically generated with a tool.
 // See script/api_protobuf/api_protobuf.py
 """
 
+# Populated by main() before any TypeInfo creation.
+# Maps enum type name (e.g. ".BluetoothDeviceRequestType") to max enum value.
+_enum_max_values: dict[str, int] = {}
+
 
 def indent_list(text: str, padding: str = "  ") -> list[str]:
     """Indent each line of the given text with the specified padding."""
@@ -1312,6 +1316,14 @@ class EnumType(TypeInfo):
     default_value = ""
     wire_type = WireType.VARINT  # Uses wire type 0
 
+    @property
+    def max_value(self) -> int | None:
+        """Get max_value from explicit annotation or auto-derive from enum definition."""
+        explicit = super().max_value
+        if explicit is not None:
+            return explicit
+        return _enum_max_values.get(self._field.type_name)
+
     @property
     def encode_func(self) -> str:
         return "encode_uint32"
@@ -1334,6 +1346,9 @@ class EnumType(TypeInfo):
         return f"static_cast<{self.cpp_type}>({value})"
 
     def get_size_calculation(self, name: str, force: bool = False) -> str:
+        max_val = self.max_value
+        if max_val is not None and max_val < 128:
+            return self._get_single_byte_varint_size(name, force)
         return self._get_simple_size_calculation(
             name, force, "uint32", f"static_cast<uint32_t>({name})"
         )
@@ -2771,6 +2786,11 @@ def main() -> None:
 
     file = d.file[0]
 
+    # Build enum max value map so EnumType can auto-derive max_value
+    for enum in file.enum_type:
+        if not enum.options.deprecated and enum.value:
+            _enum_max_values[f".{enum.name}"] = max(v.number for v in enum.value)
+
     # Build dynamic ifdef mappings early so we can emit USE_API_VARINT64 before includes
     enum_ifdef_map, message_ifdef_map, message_source_map, used_messages = (
         build_type_usage_map(file)