Fix JSONDecodeError in qwen3_coder tool parser

[BrunoCerberus]

Summary

Fixes #912

• _convert_param_value() crashes with JSONDecodeError when the model outputs object/array parameters with single quotes instead of valid JSON
• Replace the bare json.loads() + separate ast.literal_eval() paths with a single try/except that falls back gracefully

Change

# Before (crashes on single-quoted dicts)
if param_type in _obj_types or ...:
    return json.loads(param_value)
return ast.literal_eval(param_value)

# After (falls back to ast.literal_eval)
try:
    return json.loads(param_value)
except (json.JSONDecodeError, ValueError):
    return ast.literal_eval(param_value)

Test

Reproduced by running Qwen3-Coder-30B-A3B-Instruct via mlx_lm.server with tool-calling requests. The model intermittently outputs {'key': 'value'} instead of {"key": "value"} for object parameters, crashing the server. After this fix the server handles both formats.