fix bug: serialize Qwen tool argument JSON literals

[Zhijun-Xu]

Qwen's native chat template serializes non-string tool arguments as JSON. However, the Swift formatter only applied JSON serialization to dictionaries and lists, causing booleans and null values to use Python literals.

For example:

{"enabled": true, "optional": null}

Previously rendered as:

<parameter=enabled>
True
</parameter>
<parameter=optional>
None
</parameter>

This PR renders them using the expected JSON literals:

<parameter=enabled>
true
</parameter>
<parameter=optional>
null
</parameter>

The change only extends the existing JSON serialization path to bool and None. Other argument types retain their previous behavior.