fix: ensure single system message and merge historical system prompts

[728100660]

Problem

Some LLM APIs (e.g. OpenAI/Claude-compatible) require that:

• Only one system message exists
• The system message must be the first message

However, the current implementation directly prepends a new system
message while keeping existing ones from messages, which may lead to:

• Multiple system messages
• System message not at the first position

This causes errors such as:
"System message must be at the beginning"

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Real Scenario

This issue was observed when integrating with Feishu human-in-the-loop workflows.

During agent trigger execution (_invoke_agent_for_triggers),
messages may already contain system prompts constructed from previous
interaction steps (e.g. workflow instructions or injected context).

When calling the LLM again, a new system prompt is prepended without
handling the existing ones, leading to multiple system messages and API errors.

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Solution

• Extract the last system message from input messages

• Merge it with the constructed system_prompt

• Use a prefix-aware merge strategy:

  • Detect the longest common prefix (from the start)
  • Keep shared content only once
  • Append the remaining parts

• Ensure:

  • Only one system message exists
  • It is always placed at the first position

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Benefits

• Prevents API errors caused by invalid message ordering
• Fixes real issues in Feishu human-in-the-loop trigger scenarios
• Avoids duplicated prompt content
• Preserves additional instructions from historical context
• Keeps implementation simple and efficient

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Notes

• Does not introduce complex diff/LCS logic
• Follows existing code structure
• Safe fallback when no meaningful prefix match exists