Partially finished the unit test for file handler

amqp/adapter/file_handler.py

@@ -153,6 +153,11 @@ class StreamingFileHandler(FileHandler):
     def __init__(self):
         super().__init__()
 
+    # This method MUST NOT be static.
+    # For testing purpose, we will mock this method and inject a mock connection.
+    async def _get_connection(self, rabbitmq_url, loop):
+        return await connect_robust(rabbitmq_url, loop=loop)
+
     async def download_file(
         self, loop, rabbitmq_url, file_info, message_data, output_dir
     ) -> (int, int):
@@ -185,7 +190,10 @@ class StreamingFileHandler(FileHandler):
         )
 
         async def wrap_rabbit_mq_calls() -> tuple[int, int] | None:
-            connection = await connect_robust(rabbitmq_url, loop=loop)
+            # Here the normal mock doesn't work since this method
+            # is async and will be run in another loop,
+            # out of the scope of method mock.
+            connection = await self._get_connection(rabbitmq_url, loop=loop)
             try:
                 _file_size = 0
                 _eof = IN_PROGRESS