Fix task.copy() to preserve NOT_SPECIFIED context (fixes #3691)

- Add check in task.copy() to preserve NOT_SPECIFIED context value
- Previously, NOT_SPECIFIED was being converted to None during copy
- This affected kickoff_for_each and kickoff_for_each_async methods
- Add comprehensive tests covering NOT_SPECIFIED, list, and None contexts
- Rename import 'copy' to 'shallow_copy' for clarity

Co-Authored-By: João <joao@crewai.com>

src/crewai/task.py

@@ -7,7 +7,7 @@ import uuid
 import warnings
 from collections.abc import Callable
 from concurrent.futures import Future
-from copy import copy
+from copy import copy as shallow_copy
 from hashlib import md5
 from pathlib import Path
 from typing import (
@@ -671,17 +671,20 @@ Follow these guidelines:
         copied_data = self.model_dump(exclude=exclude)
         copied_data = {k: v for k, v in copied_data.items() if v is not None}
 
-        cloned_context = (
-            [task_mapping[context_task.key] for context_task in self.context]
-            if isinstance(self.context, list)
-            else None
-        )
+        if self.context is NOT_SPECIFIED:
+            cloned_context = self.context
+        else:
+            cloned_context = (
+                [task_mapping[context_task.key] for context_task in self.context]
+                if isinstance(self.context, list)
+                else None
+            )
 
         def get_agent_by_role(role: str) -> Union["BaseAgent", None]:
             return next((agent for agent in agents if agent.role == role), None)
 
         cloned_agent = get_agent_by_role(self.agent.role) if self.agent else None
-        cloned_tools = copy(self.tools) if self.tools else []
+        cloned_tools = shallow_copy(self.tools) if self.tools else []
 
         return self.__class__(
             **copied_data,