[GH-11772] Fix remaining CI failures: scope validation to POM packaging only

- Restrict consumer POM model version validation to POM-packaged
  projects only. Non-POM projects (jar, war, etc.) are consumed as
  dependencies where Maven 3 ignores unknown elements, so a higher
  model version is acceptable.
- Restore original expected error message in MavenITgh11456 mixin test
  to match the restored mixin-specific check.

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

Author: gnodet

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java

@@ -145,10 +145,13 @@ public Model build(RepositorySystemSession session, MavenProject project, ModelS
                 return buildBomWithoutFlatten(session, project, src);
             } else {
                 Model result = buildPom(session, project, src);
-                // Validate: if flattening is disabled and the consumer POM cannot be downgraded
-                // to 4.0.0, the consumer POM will contain features that Maven 3 / Gradle cannot
-                // understand. Reject the build with actionable guidance.
-                if (!model.isPreserveModelVersion()
+                // Validate POM-packaged projects (parent POMs): if the consumer POM cannot be
+                // downgraded to 4.0.0, Maven 3 / Gradle cannot resolve the parent.
+                // Non-POM projects are consumed as dependencies where unknown elements are
+                // ignored, so a higher model version is acceptable (only a warning is logged
+                // by transformNonPom/transformPom).
+                if (POM_PACKAGING.equals(packaging)
+                        && !model.isPreserveModelVersion()
                         && !ModelBuilder.MODEL_VERSION_4_0_0.equals(result.getModelVersion())) {
                     throw new MavenException("The consumer POM for " + project.getId()
                             + " cannot be downgraded to model version 4.0.0 because it contains"

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh11456MixinsConsumerPomTest.java

@@ -52,7 +52,7 @@ void testMixinsWithoutFlattening() throws Exception {
             // Expected to fail due to mixins without flattening
         }
 
-        verifier.verifyTextInLog("cannot be downgraded to model version 4.0.0");
+        verifier.verifyTextInLog("cannot be created because the POM contains mixins");
         verifier.verifyTextInLog("maven.consumer.pom.flatten=true");
     }