Track input-method contexts by identity

- deactivate only drops the context it names; one replaced by an activate
  is kept until KWin deactivates it
- Moving focus straight from one field to another ends a manual hide

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

tests/tst_visibility.cpp

@@ -27,6 +27,15 @@ private Q_SLOTS:
         QVERIFY(v.isVisible());
     }
 
+    void dismissEndsWhenAnotherFieldIsFocused()
+    {
+        VisibilityController v;
+        v.setFieldActive(true);
+        v.hide();
+        v.setFieldActive(true); // activated straight from one field to the next
+        QVERIFY(v.isVisible());
+    }
+
     void alwaysVisibleShowsWithoutField()
     {
         VisibilityController v;