[api] Drop the unused base decode(), group the vtable asserts, and test the decode emitters

A generated decodable message that lost its decode() would now fail to compile instead of
silently keeping its defaults. The asserts cover exactly the classes that derive from
ProtoDecodableMessage, under one ifdef per run instead of one per line, and the generator
tests pin the inline decode() wrapper, its absence on fixed vector messages, and the
this-> free static body.

script/api_protobuf/api_protobuf.py

@@ -2536,8 +2536,8 @@ def build_message_type(
 
     # Get source direction to determine if we need decode/encode methods
     source = message_source_map[desc.name]
-    needs_decode = source in (SOURCE_BOTH, SOURCE_CLIENT)
-    needs_encode = source in (SOURCE_BOTH, SOURCE_SERVER)
+    needs_decode = message_needs_decode(source)
+    needs_encode = message_needs_encode(source)
 
     # Add MESSAGE_TYPE method if this is a service message
     if message_id is not None:
@@ -2846,6 +2846,23 @@ def get_field_opt(
     return field.options.Extensions[opt]
 
 
+def message_needs_decode(source: int) -> bool:
+    return source in (SOURCE_BOTH, SOURCE_CLIENT)
+
+
+def message_needs_encode(source: int) -> bool:
+    return source in (SOURCE_BOTH, SOURCE_SERVER)
+
+
+def is_decodable_class(desc: descriptor.DescriptorProto, source: int) -> bool:
+    """Whether the generated class derives from ProtoDecodableMessage: decoded, and either on a
+    decodable base class or with at least one live field."""
+    return message_needs_decode(source) and (
+        get_base_class(desc) is not None
+        or any(not field.options.deprecated for field in desc.field)
+    )
+
+
 def get_base_class(desc: descriptor.DescriptorProto) -> str | None:
     """Get the base_class option from a message descriptor."""
     if not desc.options.HasExtension(pb.base_class):
@@ -3402,7 +3419,7 @@ static void dump_bytes_field(DumpBuffer &out, const char *field_name, const uint
             continue
 
         s, c, dc = build_message_type(m, base_class_fields, message_source_map)
-        if message_source_map[m.name] in (SOURCE_BOTH, SOURCE_CLIENT):
+        if is_decodable_class(m, message_source_map[m.name]):
             decodable_messages.append((m.name, message_ifdef_map.get(m.name)))
         msg_ifdef = message_ifdef_map.get(m.name)
 
@@ -3433,9 +3450,17 @@ static void dump_bytes_field(DumpBuffer &out, const char *field_name, const uint
     # decode() passes decode_field explicitly, so without the dump virtuals no decodable message
     # may carry a vtable; a build at any level below VERY_VERBOSE proves it
     cpp += "#ifndef HAS_PROTO_MESSAGE_DUMP\n"
+    assert_ifdef = None
     for name, msg_ifdef in decodable_messages:
-        line = f'static_assert(!std::is_polymorphic_v<{name}>, "decodable messages carry no vtable");'
-        cpp += "\n".join(wrap_with_ifdef(line, msg_ifdef)) + "\n"
+        if msg_ifdef != assert_ifdef:
+            if assert_ifdef is not None:
+                cpp += "#endif\n"
+            if msg_ifdef is not None:
+                cpp += _make_ifdef_line(msg_ifdef) + "\n"
+            assert_ifdef = msg_ifdef
+        cpp += f'static_assert(!std::is_polymorphic_v<{name}>, "decodable messages carry no vtable");\n'
+    if assert_ifdef is not None:
+        cpp += "#endif\n"
     cpp += "#endif\n"
 
     content += """\