Add XML formatting and syntax highlighting support

[yizzy]

Please add XML formatting and syntax highlighting support to this project.

Detailed Requirements:

1. Add a format_xml boolean configuration option under the response section in the config schema.
2. Create an XML utility function (prefer standard library xml.dom.minidom) to pretty-print raw XML strings. Ensure malformed XML safely falls back to returning the raw text.
3. Detect when a response's Content-Type header includes application/xml or text/xml. If format_xml is enabled, format the response body before passing it to the UI component.
4. Ensure the Textual TextArea UI component sets syntax highlighting to "xml" for XML responses.
5. Add a "Format XML" command to the command palette so users can format XML in the request body editor.
6. Add unit tests in the tests/ directory verifying the XML formatting utility and Content-Type matching logic.