feat: add AgentMaxIterationError exc

[fatelei]

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Summary

fix #30419

add AgentMaxIterationError exc

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new exception, AgentMaxIterationError, to improve the robustness of agent execution. It prevents agents from entering infinite loops by explicitly raising an error when they exceed their configured maximum iteration limit while still attempting to invoke tools. This ensures more predictable behavior and clearer error reporting for agent-based workflows.

Highlights

• New Exception for Agent Iteration Limits: Introduced AgentMaxIterationError to specifically handle scenarios where an agent exceeds its maximum allowed iterations.
• Enhanced Agent Loop Detection: Implemented checks in both cot_agent_runner.py and fc_agent_runner.py to raise AgentMaxIterationError when an agent reaches its iteration limit and attempts further tool calls instead of concluding.

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[gemini-code-assist]

Code Review

This pull request introduces a new AgentMaxIterationError exception to handle scenarios where an agent exceeds its configured maximum number of iterations. The exception is raised in both CotAgentRunner and FunctionCallAgentRunner when they detect that the agent is attempting to continue processing beyond the iteration limit instead of providing a final answer. The implementation is clean and effectively addresses the problem. I have one minor suggestion to improve code readability by simplifying a conditional check.