fix: handle 'Action: None' in parser to prevent OutputParserError

[devin-ai-integration]

Summary

Fixes #4186 - When LLMs output Action: None (or variations like Action: N/A, Action: no action), the parser now correctly treats this as a signal for a direct response instead of raising an OutputParserError.

Previously, the ReAct parser would fail when an LLM indicated it couldn't/shouldn't use a tool by outputting Action: None, causing the error handler to leak internal "Thought:" text to users.

Changes:

• Added ACTION_NONE_REGEX constant to match non-action values (none, n/a, na, no action, no_action)
• Updated parse() to detect Action: None patterns and convert them to AgentFinish
• The thought content becomes the output, providing a clean response to users
• Added 11 tests covering various formats

Review & Testing Checklist for Human

• Verify regex doesn't match valid tool names: The pattern matches none|n/a|na|no action|no_action case-insensitively. Confirm no legitimate tools could be named this way.
• Test with real LLM outputs: Run a crew with allow_delegation=True but no other agents to trigger the original issue scenario and verify the fix works end-to-end.
• Review parenthesis handling logic (parser.py lines 130-134): The logic for stripping parentheses from patterns like Action: None (reason) has some complexity that may have edge cases.

Suggested test plan:

from crewai import Agent, Task, Crew

agent = Agent(
    role="Manager",
    goal="Help users",
    backstory="You coordinate tasks",
    allow_delegation=True
)

task = Task(
    description="Find information about XYZ integration",
    expected_output="Information about XYZ",
    agent=agent
)

crew = Crew(agents=[agent], tasks=[task])
result = crew.kickoff()
# Verify result doesn't contain "Thought:" prefix

Notes

• Link to Devin run: https://app.devin.ai/sessions/060a7c144a4c4e4ab38ad88f9958d02b
• Requested by: João (joao@crewai.com)

[devin-ai-integration]

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