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>

tests/test_task.py

@@ -900,6 +900,59 @@ def test_conditional_task_copy_preserves_type():
     assert isinstance(copied_conditional_task, ConditionalTask)
 
 
+def test_task_copy_preserves_not_specified_context():
+    """Test that copying a task preserves NOT_SPECIFIED context value."""
+    from crewai.utilities.constants import NOT_SPECIFIED
+
+    task = Task(
+        description="Test task",
+        expected_output="Test output"
+    )
+
+    assert task.context is NOT_SPECIFIED
+
+    copied_task = task.copy(agents=[], task_mapping={})
+
+    assert copied_task.context is NOT_SPECIFIED
+    assert copied_task.context is not None
+
+
+def test_task_copy_with_list_context():
+    """Test that copying a task with list context works correctly."""
+    task1 = Task(
+        description="Task 1",
+        expected_output="Output 1"
+    )
+    task2 = Task(
+        description="Task 2",
+        expected_output="Output 2",
+        context=[task1]
+    )
+
+    task_mapping = {task1.key: task1}
+
+    copied_task2 = task2.copy(agents=[], task_mapping=task_mapping)
+
+    assert isinstance(copied_task2.context, list)
+    assert len(copied_task2.context) == 1
+    assert copied_task2.context[0] is task1
+
+
+def test_task_copy_with_none_context():
+    """Test that copying a task with None context works correctly."""
+    task = Task(
+        description="Test task",
+        expected_output="Test output",
+        context=None
+    )
+
+    assert task.context is None
+
+    copied_task = task.copy(agents=[], task_mapping={})
+
+    assert copied_task.context is None
+
+
 def test_interpolate_inputs(tmp_path):
     task = Task(
         description="Give me a list of 5 interesting ideas about {topic} to explore for an article, what makes them unique and interesting.",