fix: resolve type annotation mismatch in Task.context field

- Update context field type annotation from Optional[List[Task]] to Union[List[Task], _NotSpecified, None]
- Add ConfigDict(arbitrary_types_allowed=True) to Task model to support _NotSpecified type
- Add comprehensive tests covering the type annotation fix and sentinel behavior
- Fixes issue #3019 where context field default NOT_SPECIFIED didn't match type annotation

The fix maintains backward compatibility while making the type annotation accurate.
The NOT_SPECIFIED sentinel distinguishes between 'not passed' and 'explicitly passed None'.

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

src/crewai/task.py

@@ -26,6 +26,7 @@ from typing import (
 from pydantic import (
     UUID4,
     BaseModel,
+    ConfigDict,
     Field,
     PrivateAttr,
     field_validator,
@@ -39,7 +40,7 @@ from crewai.tasks.output_format import OutputFormat
 from crewai.tasks.task_output import TaskOutput
 from crewai.tools.base_tool import BaseTool
 from crewai.utilities.config import process_config
-from crewai.utilities.constants import NOT_SPECIFIED
+from crewai.utilities.constants import NOT_SPECIFIED, _NotSpecified
 from crewai.utilities.guardrail import process_guardrail, GuardrailResult
 from crewai.utilities.converter import Converter, convert_to_model
 from crewai.utilities.events import (
@@ -78,6 +79,8 @@ class Task(BaseModel):
     used_tools: int = 0
     tools_errors: int = 0
     delegations: int = 0
+    
+    model_config = ConfigDict(arbitrary_types_allowed=True)
     i18n: I18N = I18N()
     name: Optional[str] = Field(default=None)
     prompt_context: Optional[str] = None
@@ -95,7 +98,7 @@ class Task(BaseModel):
     agent: Optional[BaseAgent] = Field(
         description="Agent responsible for execution the task.", default=None
     )
-    context: Optional[List["Task"]] = Field(
+    context: Union[List["Task"], _NotSpecified, None] = Field(
         description="Other tasks that will have their output used as context for this task.",
         default=NOT_SPECIFIED,
     )