Unstable test 'test_CREATE_inside'

See https://github.com/eclipse-windowbuilder/windowbuilder/pull/1347/checks?check_run_id=60402436562

```java
expected: but was:
org.opentest4j.AssertionFailedError:
expected: but was:
at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:182)
at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:177)
at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1145)
at org.eclipse.wb.tests.designer.core.AbstractJavaTest.assertEditor(AbstractJavaTest.java:273)
at org.eclipse.wb.tests.designer.swing.SwingGefTest.assertEditor(SwingGefTest.java:115)
at org.eclipse.wb.tests.designer.editor.validator.BorderOfChildLayoutRequestValidatorTest.test_CREATE_inside(BorderOfChildLayoutRequestValidatorTest.java:59)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at org.eclipse.wb.tests.designer.core.model.parser.AbstractJavaInfoTest$JavaInfoExtension.interceptTestMethod(AbstractJavaInfoTest.java:85)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
Jan. 14, 2026 8:44:41 PM org.eclipse.wb.tests.designer.tests.DesignerTestCase$LoggerExtension beforeTestExecution
```

Comienza con BorderOfChildLayoutRequestValidatorTest.test_CREATE_inside, al que se hace referencia en el fallo de las comprobaciones del pull request, y reproduce la prueba para determinar por qué la salida del editor generado omite el JButton. Compara el código fuente Java esperado y el real, y confirma que la prueba pasa de forma consistente una vez abordada la inestabilidad subyacente.

Escrito por el modelo de indexación a partir del texto del issue.