refactor(backend): implement default value handling in serialization

The `json_serialize` method has been added to the `TelegramMethod` and
`TelegramObject` classes to replace `Default` placeholders with actual
values from the bot's defaults during JSON serialization. This change
ensures that non-standard objects are handled correctly, maintaining
backward compatibility for built-in pydantic json serialization.

This modification is beneficial as it centralizes the handling of
default values when serializing objects to JSON, making the code more
maintainable and robust against future changes in serialization logic.

tests/test_api/test_methods/test_base.py

@@ -1,9 +1,11 @@
+from typing import Any, Dict
 from unittest.mock import sentinel
 
 import pytest
 
-from aiogram.methods import GetMe, TelegramMethod
-from aiogram.types import TelegramObject, User
+from aiogram.client.default import Default
+from aiogram.methods import GetMe, SendMessage, TelegramMethod
+from aiogram.types import LinkPreviewOptions, TelegramObject, User
 from tests.mocked_bot import MockedBot
 
 
@@ -26,6 +28,25 @@ class TestTelegramMethodRemoveUnset:
         assert obj.remove_unset("") == ""
 
 
+class TestTelegramMethodJsonSerialize:
+    @pytest.mark.parametrize(
+        "obj",
+        [
+            SendMessage(
+                chat_id=1,
+                text="test",
+            ),
+            LinkPreviewOptions(),
+        ],
+    )
+    def test_json_serialize(self, obj):
+        def has_defaults(dump: Dict[str, Any]) -> bool:
+            return any(isinstance(value, Default) for value in dump.values())
+
+        assert has_defaults(obj.model_dump())
+        assert not has_defaults(obj.model_dump(mode="json"))
+
+
 class TestTelegramMethodCall:
     async def test_async_emit_unsuccessful(self, bot: MockedBot):
         with pytest.raises(