fix: clear agent executor messages when reused across tasks

[devin-ai-integration]

Summary

Fixes #4319

When the same agent is reused across multiple sequential tasks (common pattern in Flow with @listen decorators), the agent executor's message history was not being cleared between tasks. This caused messages to accumulate, leading to duplicate system messages, context pollution, and eventually crashes with "Invalid response from LLM call - None or empty".

The fix adds two lines to _update_executor_parameters() in agent/core.py to clear the messages list and reset the iterations counter when the agent executor is reused for a new task.

Review & Testing Checklist for Human

• Verify clearing messages is always correct: Confirm there's no scenario where preserving message history between tasks is intentional behavior. The fix unconditionally clears messages whenever _update_executor_parameters is called.
• Test with real Flow pattern: The unit tests verify state is cleared but don't run actual LLM calls. Recommend testing with a real Flow using @listen decorators where the same agent executes multiple sequential tasks.
• Check for regressions in existing agent reuse patterns: Run any existing integration tests that involve agent reuse across crews/tasks.

Suggested manual test:

from crewai import Agent, Task, Crew

agent = Agent(role="Test", goal="Test", backstory="Test", allow_delegation=False)

for i in range(3):
    task = Task(description=f"Task {i}", expected_output="Result", agent=agent)
    crew = Crew(agents=[agent], tasks=[task])
    result = crew.kickoff()
    print(f"Task {i} completed, messages count: {len(agent.agent_executor.messages)}")

Messages should reset to a consistent count after each task, not accumulate.

Notes

• Link to Devin run: https://app.devin.ai/sessions/267b9e4dac04463c9c70faf692a950a0
• Requested by: João

[devin-ai-integration]

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