chat : fix LFM2 tool-call parsing double-escaping

[tdakhran]

Overview

Prevent double escaping in the LFM2 tool-calling parser

Output before fix:

    {
      "type": "function",
      "function": {
        "name": "python",
        "arguments": "{\"code\":\"print('hello')\\\\nprint('world')\"}"
      }
    }

Output after fix:

    {
      "type": "function",
      "function": {
        "name": "python",
        "arguments": "{\"code\":\"print('hello')\\nprint('world')\"}"
      }
    }

Additional information

Add extra test cases

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO