feat: expose messages to TaskOutput and LiteAgentOutputs (#3880)

* feat: add messages to task and agent outputs

- Introduced a new  field in  and  to capture messages from the last task execution.
- Updated the  class to store the last messages and provide a property for easy access.
- Enhanced the  and  classes to include messages in their outputs.
- Added tests to ensure that messages are correctly included in task outputs and agent outputs during execution.

* using typing_extensions for 3.10 compatability

* feat: add last_messages attribute to agent for improved task tracking

- Introduced a new `last_messages` attribute in the agent class to store messages from the last task execution.
- Updated the `Crew` class to handle the new messages attribute in task outputs.
- Enhanced existing tests to ensure that the `last_messages` attribute is correctly initialized and utilized across various guardrail scenarios.

* fix: add messages field to TaskOutput in tests for consistency

- Updated multiple test cases to include the new `messages` field in the `TaskOutput` instances.
- Ensured that all relevant tests reflect the latest changes in the TaskOutput structure, maintaining consistency across the test suite.
- This change aligns with the recent addition of the `last_messages` attribute in the agent class for improved task tracking.

* feat: preserve messages in task outputs during replay

- Added functionality to the Crew class to store and retrieve messages in task outputs.
- Enhanced the replay mechanism to ensure that messages from stored task outputs are preserved and accessible.
- Introduced a new test case to verify that messages are correctly stored and replayed, ensuring consistency in task execution and output handling.
- This change improves the overall tracking and context retention of task interactions within the CrewAI framework.

* fix original test, prev was debugging

lib/crewai/tests/agents/test_lite_agent.py

@@ -238,6 +238,27 @@ def test_lite_agent_returns_usage_metrics():
     assert result.usage_metrics["total_tokens"] > 0
 
 
+@pytest.mark.vcr(filter_headers=["authorization"])
+def test_lite_agent_output_includes_messages():
+    """Test that LiteAgentOutput includes messages from agent execution."""
+    llm = LLM(model="gpt-4o-mini")
+    agent = Agent(
+        role="Research Assistant",
+        goal="Find information about the population of Tokyo",
+        backstory="You are a helpful research assistant who can search for information about the population of Tokyo.",
+        llm=llm,
+        tools=[WebSearchTool()],
+        verbose=True,
+    )
+
+    result = agent.kickoff("What is the population of Tokyo?")
+
+    assert isinstance(result, LiteAgentOutput)
+    assert hasattr(result, "messages")
+    assert isinstance(result.messages, list)
+    assert len(result.messages) > 0
+
+
 @pytest.mark.vcr(filter_headers=["authorization"])
 @pytest.mark.asyncio
 async def test_lite_agent_returns_usage_metrics_async():