Add context manager support for bot client (#1468)

* Add context manager support for bot client

The bot client now supports the context manager protocol, providing automatic resource management. This enhancement helps to automatically close the session when leaving the context, which cleans up resources better. The documentation and tests have been updated accordingly to illustrate this new feature. Moreover, an example of usage without a dispatcher has been provided to clarify its use in simple cases.

* Added changelog

aiogram/client/bot.py

@@ -5,6 +5,7 @@ import io
 import pathlib
 import warnings
 from contextlib import asynccontextmanager
+from types import TracebackType
 from typing import (
     Any,
     AsyncGenerator,
@@ -12,6 +13,7 @@ from typing import (
     BinaryIO,
     List,
     Optional,
+    Type,
     TypeVar,
     Union,
     cast,
@@ -300,6 +302,17 @@ class Bot:
         self.__token = token
         self._me: Optional[User] = None
 
+    async def __aenter__(self) -> "Bot":
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_value: Optional[BaseException],
+        traceback: Optional[TracebackType],
+    ) -> None:
+        await self.session.close()
+
     @property
     def parse_mode(self) -> Optional[str]:
         warnings.warn(