Keep the user's shift; trust the text guess less

- A tapped shift (on or off) survives the field re-reporting its context:
  it lasts until a character is typed, the hint changes or a field is focused
- An input-method reset forgets the guessed text and pending dead key, so
  double-space can't delete a character the keyboard never saw
- Fields that state typing preferences only get capitals if they ask
- Trackpad moves in fields without surrounding text make the guess unknown;
  Backspace removes a whole grapheme cluster from it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/tst_textrules.cpp

@@ -44,6 +44,9 @@ private Q_SLOTS:
     {
         QCOMPARE(TextRules::withoutLastCharacter(u"ab"_s), u"a");
         QCOMPARE(TextRules::withoutLastCharacter(u"a👍"_s), u"a");
+        QCOMPARE(TextRules::withoutLastCharacter(u"ae\u0301"_s), u"a"); // e + combining acute
+        QCOMPARE(TextRules::withoutLastCharacter(u"a👍🏽"_s), u"a"); // with a skin tone
+        QCOMPARE(TextRules::withoutLastCharacter(QString()), QString());
         QCOMPARE(TextRules::withoutLastWord(u"one two  "_s), u"one ");
         QCOMPARE(TextRules::withoutLastWord(u"one"_s), QString());
     }