feat: add XML and HTML formatting support (#721)

test/xml_format_test.dart

@@ -0,0 +1,46 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:proxypin/network/http/content_type.dart';
+import 'package:proxypin/network/http/http.dart';
+import 'package:proxypin/utils/xml_formatter.dart';
+
+void main() {
+  test('XML.pretty formats nested document', () {
+    const input = '<root><a>1</a><b><c>2</c></b></root>';
+
+    expect(
+      XML.pretty(input),
+      '<root>\n'
+      '  <a>1</a>\n'
+      '  <b>\n'
+      '    <c>2</c>\n'
+      '  </b>\n'
+      '</root>',
+    );
+  });
+
+  test('XML.pretty falls back for malformed XML', () {
+    const input = '<root><a></root>';
+    expect(XML.pretty(input), input);
+  });
+
+  test('xml content types map to ContentType.xml', () {
+    final response = HttpResponse(HttpStatus.ok);
+
+    response.headers.set('content-type', 'application/xml; charset=utf-8');
+    expect(response.contentType, ContentType.xml);
+
+    response.headers.set('content-type', 'text/xml; charset=utf-8');
+    expect(response.contentType, ContentType.xml);
+
+    response.headers.set('content-type', 'application/soap+xml; charset=utf-8');
+    expect(response.contentType, ContentType.xml);
+  });
+
+  test('xhtml keeps html content type', () {
+    final response = HttpResponse(HttpStatus.ok);
+    response.headers.set('content-type', 'application/xhtml+xml; charset=utf-8');
+
+    expect(response.contentType, ContentType.html);
+  });
+}
+