fix: guard QuotedPrintable decode against truncated =XY escapes

lib/src/codecs/quoted_printable_mail_codec.dart

@@ -197,6 +197,14 @@ class QuotedPrintableMailCodec extends MailCodec {
     for (var i = 0; i < cleaned.length; i++) {
       final char = cleaned[i];
       if (char == '=') {
+        // A '=' within two characters of the end has no complete hex escape
+        // following it. RFC 2045 §6.7 permits consumers to preserve such
+        // malformed input verbatim; the alternative here is a RangeError
+        // from substring on the two lines below.
+        if (i + 3 > cleaned.length) {
+          buffer.write(cleaned.substring(i));
+          break;
+        }
         final hexText = cleaned.substring(i + 1, i + 3);
         var charCode = int.tryParse(hexText, radix: 16);
         if (charCode == null) {
@@ -208,6 +216,11 @@ class QuotedPrintableMailCodec extends MailCodec {
         } else {
           final charCodes = [charCode];
           while (cleaned.length > (i + 4) && cleaned[i + 3] == '=') {
+            // The while check only proves cleaned[i+3] exists; the substring
+            // below still reads cleaned[i+4..i+5], so stop if the trailing
+            // '=' has fewer than two hex chars after it and let the outer
+            // loop pick it up via the truncated-tail branch above.
+            if (i + 6 > cleaned.length) break;
             i += 3;
             final hexText = cleaned.substring(i + 1, i + 3);
             charCode = int.parse(hexText, radix: 16);

test/codecs/quoted_printable_mail_codec_test.dart

@@ -74,6 +74,37 @@ void main() {
         'jeden Tag ändern können',
       );
     });
+
+    group('malformed truncated =XY escapes are preserved verbatim', () {
+      // Regression: a '=' within two characters of the end used to throw
+      // RangeError from substring(i+1, i+3). Per RFC 2045 §6.7 consumers may
+      // preserve the malformed tail; matches the existing behaviour for
+      // non-hex '=XY' where the raw text is written to the buffer.
+      test('trailing bare "="', () {
+        expect(
+          MailCodec.quotedPrintable.decodeText('hello=', convert.utf8),
+          'hello=',
+        );
+      });
+      test('trailing "=A"', () {
+        expect(
+          MailCodec.quotedPrintable.decodeText('hello=A', convert.utf8),
+          'hello=A',
+        );
+      });
+      test('lone "=A" at end of a short input', () {
+        expect(
+          MailCodec.quotedPrintable.decodeText('h=A', convert.utf8),
+          'h=A',
+        );
+      });
+      test('valid multi-byte escape followed by truncated "=A"', () {
+        expect(
+          MailCodec.quotedPrintable.decodeText('hi=C3=B6=A', convert.utf8),
+          'hiö=A',
+        );
+      });
+    });
   });
 
   group('Quoted Printable encoding', () {