fix(api): fix incorrect adjust to Extensible

`__file__` records the path of the `Extensible` class, not the receiver class
of `scan_extensions` method.

This commit fixes this by switching to `inspect.getfile`.

api/core/extension/extensible.py

@@ -1,5 +1,6 @@
 import enum
 import importlib.util
+import inspect
 import json
 import logging
 import os
@@ -42,7 +43,7 @@ class Extensible:
         position_map: dict[str, int] = {}
 
         # get the path of the current class
-        current_path = os.path.abspath(__file__)
+        current_path = os.path.abspath(inspect.getfile(cls))
         current_dir_path = os.path.dirname(current_path)
 
         # traverse subdirectories