fix: Update session clearing condition in SessionContextHolder

- Changed the condition for clearing the session from checking if the depth is 0 to using TransactionalDepth.ON_EXIT.value for better clarity and alignment with transaction management.

Author: William-W-Chen

py_spring_model/core/session_context_holder.py

@@ -119,7 +119,7 @@ def exit_session(cls) -> int:
         cls._session_depth.set(new_depth)
 
         # Clear session only when depth reaches 0 (outermost level)
-        if new_depth == 0:
+        if new_depth == TransactionalDepth.ON_EXIT.value:
             cls.clear_session()
 
         return new_depth