[api] Trim the decode path: void decode_field, bool from the low word, slow varint out parameter

decode_field() no longer returns a bool that only fed a verbose log; unknown fields are
skipped silently like every other protobuf decoder does, and each message loses the
return value materialisation. Bools read the low 32 bits of the varint, which drops the
second compare on 64 bit varint builds. The multi byte varint path writes its value
through an out parameter instead of returning a 16 byte struct, which takes the spills
out of the decode loop and count_repeated_field.

script/api_protobuf/api_protobuf.py

@@ -804,7 +804,7 @@ class BoolType(VarintTypeMixin, TypeInfo):
     _varint_max_bits = 1
     cpp_type = "bool"
     default_value = "false"
-    decode_expr = "value.as_varint() != 0"
+    decode_expr = "value.as_bool()"
     encode_func = "encode_bool"
     wire_type = WireType.VARINT  # Uses wire type 0
 
@@ -2658,16 +2658,16 @@ def build_message_type(
 
     cpp = ""
     if decode:
-        o = f"bool {desc.name}::decode_field(uint32_t tag, const uint8_t *data, proto_varint_value_t scalar) {{\n"
+        o = f"void {desc.name}::decode_field(uint32_t tag, const uint8_t *data, proto_varint_value_t scalar) {{\n"
         o += "  const ProtoFieldValue value(data, scalar);\n"
         o += "  switch (tag) {\n"
         o += indent("\n".join(decode), "    ") + "\n"
-        o += "    default: return false;\n"
+        o += "    default:\n"
+        o += "      break;\n"
         o += "  }\n"
-        o += "  return true;\n"
         o += "}\n"
         cpp += o
-        prot = "bool decode_field(uint32_t tag, const uint8_t *data, proto_varint_value_t scalar) override;"
+        prot = "void decode_field(uint32_t tag, const uint8_t *data, proto_varint_value_t scalar) override;"
         protected_content.insert(0, prot)
 
     # Generate custom decode() override for messages with FixedVector fields