Clarify AXIS2-6053 diagnostic hint to indicate it is one possible cause

Reword the hint to say "if this attribute is expected to be valid, one
common cause is..." rather than "This may be caused by...", making it
clear the hint is advisory and only relevant when the attribute should
have been allowed by the schema.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: robertlazarski

modules/schema-validation/src/main/java/org/apache/axis2/validation/SchemaValidationHandler.java

@@ -88,11 +88,12 @@ public InvocationResponse invoke(MessageContext msgContext) throws AxisFault {
     static String appendRefHint(SAXException ex) {
         String msg = ex.getMessage();
         if (msg != null && msg.contains("cvc-complex-type.3.2.2")) {
-            return ". This may be caused by a schema using xs:attribute ref= to reference"
+            return ". Note: if this attribute is expected to be valid,"
+                    + " one common cause is a schema using xs:attribute ref= to reference"
                     + " an attribute from an external namespace (e.g., xmime:contentType)"
                     + " whose schema was not imported or could not be resolved."
-                    + " Consider defining the attribute inline instead of using ref=,"
-                    + " or ensure the referenced schema is accessible.";
+                    + " In that case, ensure the referenced schema is accessible"
+                    + " or define the attribute inline instead of using ref=.";
         }
         return "";
     }

modules/schema-validation/src/test/java/org/apache/axis2/validation/SchemaValidationHandlerTest.java

@@ -49,8 +49,8 @@ public void testAppendRefHintForCvcComplexType322() {
         SAXException ex = new SAXException(
                 "cvc-complex-type.3.2.2: Attribute 'contentType' is not allowed to appear in element 'foo'");
         String hint = SchemaValidationHandler.appendRefHint(ex);
+        assertThat(hint).contains("if this attribute is expected to be valid");
         assertThat(hint).contains("xs:attribute ref=");
-        assertThat(hint).contains("xmime:contentType");
         assertThat(hint).contains("not imported or could not be resolved");
     }
 
@@ -128,7 +128,7 @@ public void testInvokeProducesRefHintForUnexpectedAttribute() throws Exception {
         assertThatThrownBy(() -> handler.invoke(msgCtx))
                 .isInstanceOf(AxisFault.class)
                 .hasMessageContaining("cvc-complex-type.3.2.2")
-                .hasMessageContaining("xs:attribute ref=")
-                .hasMessageContaining("not imported or could not be resolved");
+                .hasMessageContaining("if this attribute is expected to be valid")
+                .hasMessageContaining("xs:attribute ref=");
     }
 }