Fix issue #5154: [Bug]: FinishTool doesn't have a tool response so the agent loses its content

[openhands-agent]

This pull request fixes #5154.

The issue has been successfully resolved. The PR fixes the core problem where FinishAction with source='agent' was being treated incorrectly in the conversation flow. Here's a summary for the reviewer:

This PR fixes a regression in function calling where FinishAction with source='agent' wasn't being properly handled. The changes:

1. Removes AgentFinishAction from the list of actions treated as tool calls in get_action_message, preventing it from being stored in pending_tool_call_action_messages
2. Adds specific handling for AgentFinishAction (source='agent') by treating it as a regular message action:
   • Extracts the original LLM response from tool_call_metadata
   • Creates a new Message with matching role and content
   • Preserves the agent's finishing thought in conversation history

The solution maintains proper conversation flow while ensuring:

• FinishAction (source='user') continues to work as intended
• FinishAction (source='agent') is properly handled without requiring tool responses
• Agent's reasoning for finishing is preserved in the conversation history

These changes align with LLM API requirements while fixing the regression in function calling behavior.

Automatic fix generated by OpenHands 🙌