Sanitize invalid XML characters in text content

Strip invalid XML 1.0 control characters (0x00-0x08, 0x0B-0x0C, 0x0E-0x1F)
from text to prevent corrupted docx files that fail to open in LibreOffice.

Fixes SAXParseException 'PCData Invalid Char value' errors.

test/paragraph_test.rb

@@ -111,4 +111,21 @@ class ParagraphTest < Minitest::Test
     # Newlines should be preserved in the text
     assert_includes xml, "Line 1\nLine 2\nLine 3"
   end
+
+  def test_invalid_xml_characters_are_stripped
+    xml = create_doc_and_read_xml do |doc|
+      doc.p "infrastruktur\x02bidrag"
+      doc.p "hello\x00world"
+      doc.p "test\x01\x03\x04value"
+    end
+
+    # Invalid characters should be stripped
+    assert_includes xml, "infrastrukturbidrag"
+    assert_includes xml, "helloworld"
+    assert_includes xml, "testvalue"
+
+    # Verify the XML is valid by parsing it (will raise if invalid)
+    doc = Nokogiri::XML(xml, &:strict)
+    assert doc.errors.empty?, "XML should be valid: #{doc.errors}"
+  end
 end