fix: clean <answer> tags from model output in parse_action

[majiayu000]

Summary

• Add explicit removal of <answer> and </answer> tags from model output
• Use replace() instead of slice indexing for robust tag cleaning
• Add tests to verify tag cleaning

Problem

The model sometimes outputs responses wrapped in <answer> tags:

{
  "_metadata": "do",
  "action": "Type",
  "text": "文件传输助手\")</answe"
}

This causes action parsing to fail or produce malformed output.

Solution

Add replace('<answer>', '') and replace('</answer>', '') at the beginning of parse_action to clean up these tags before parsing.

Fixes #258