[GH-11772] Fix CI failures: revert build POM resolution and fix consumer POM validation

- Revert DefaultModelBuilder.java build POM resolution change (can be
  a separate PR) to avoid side effects during parent resolution
- Restore original mixin check in DefaultConsumerPomBuilder
- Remove overly aggressive validation from flatten-enabled POM path
  that was rejecting mixin-containing POMs (model version 4.2.0)
- Keep validation only on non-flatten path: consumer POMs that cannot
  be downgraded to 4.0.0 still fail fast with actionable guidance

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

@@ -122,6 +122,20 @@ public Model build(RepositorySystemSession session, MavenProject project, ModelS
         // Check if this is a BOM (original packaging is "bom")
         boolean isBom = BOM_PACKAGING.equals(originalPackaging);
 
+        // Check if mixins are present without flattening enabled
+        if (!model.getMixins().isEmpty() && !flattenEnabled && !model.isPreserveModelVersion()) {
+            throw new MavenException("The consumer POM for "
+                    + project.getId()
+                    + " cannot be created because the POM contains mixins. "
+                    + "Mixins are not supported in the default consumer POM format. "
+                    + "You have the following options to resolve this:" + System.lineSeparator()
+                    + "  1. Preserve the model version by setting 'preserve.model.version=true' to generate a consumer POM with <modelVersion>4.2.0</modelVersion>, which supports mixins"
+                    + System.lineSeparator()
+                    + "  2. Enable flattening by setting the property 'maven.consumer.pom.flatten=true' to remove mixins during transformation"
+                    + System.lineSeparator()
+                    + "  3. Remove the mixins from your POM");
+        }
+
         // Check if consumer POM flattening is disabled
         if (!flattenEnabled) {
             // When flattening is disabled, treat non-POM projects like parent POMs
@@ -133,8 +147,7 @@ public Model build(RepositorySystemSession session, MavenProject project, ModelS
                 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. Since enabling flattening would inline the parent and produce a
-                // self-contained 4.0.0 POM, reject the build with actionable guidance.
+                // understand. Reject the build with actionable guidance.
                 if (!model.isPreserveModelVersion()
                         && !ModelBuilder.MODEL_VERSION_4_0_0.equals(result.getModelVersion())) {
                     throw new MavenException("The consumer POM for " + project.getId()
@@ -159,22 +172,7 @@ public Model build(RepositorySystemSession session, MavenProject project, ModelS
             if (isBom) {
                 return buildBom(session, project, src);
             } else {
-                Model result = buildPom(session, project, src);
-                // Validate: POM-packaged projects (parents) keep the parent reference
-                // regardless of flatten setting. If the consumer POM cannot be downgraded
-                // to 4.0.0, reject the build.
-                if (!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"
-                            + " features that require a newer model version." + System.lineSeparator()
-                            + "You have the following options to resolve this:" + System.lineSeparator()
-                            + "  1. Preserve the model version by setting 'preserve.model.version=true'"
-                            + " on the <project> element (Maven 4 consumers only)"
-                            + System.lineSeparator()
-                            + "  2. Remove the features that require a newer model version");
-                }
-                return result;
+                return buildPom(session, project, src);
             }
         } else {
             return buildNonPom(session, project, src);

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelBuilder.java

@@ -1257,46 +1257,19 @@ Model resolveAndReadParentExternally(
                         ? resolveReactorModel(groupId, artifactId, version)
                         : null;
                 if (modelSource == null) {
-                    // For build requests resolving a regular parent (not mixin) from the repository,
-                    // try the "build" classifier first to get the full-fidelity POM.
-                    // The consumer POM (no classifier) may be stripped to 4.0.0 for backward
-                    // compatibility, losing 4.1.0+ features that are needed during the build.
-                    String resolveClassifier = classifier;
-                    if (resolveClassifier == null && isBuildRequestWithActivation() && !(parent instanceof Mixin)) {
-                        try {
-                            ModelResolver.ModelResolverRequest buildReq = new ModelResolver.ModelResolverRequest(
-                                    request.getSession(),
-                                    null,
-                                    repositories,
-                                    groupId,
-                                    artifactId,
-                                    version,
-                                    "build",
-                                    "pom");
-                            ModelResolver.ModelResolverResult buildResult = modelResolver.resolveModel(buildReq);
-                            modelSource = buildResult.source();
-                            if (buildResult.version() != null) {
-                                parent = parent.withVersion(buildResult.version());
-                            }
-                        } catch (ModelResolverException e) {
-                            // No build POM available, fall back to main POM
-                        }
-                    }
-                    if (modelSource == null) {
-                        ModelResolver.ModelResolverRequest req = new ModelResolver.ModelResolverRequest(
-                                request.getSession(),
-                                null,
-                                repositories,
-                                groupId,
-                                artifactId,
-                                version,
-                                resolveClassifier,
-                                extension != null ? extension : "pom");
-                        ModelResolver.ModelResolverResult result = modelResolver.resolveModel(req);
-                        modelSource = result.source();
-                        if (result.version() != null) {
-                            parent = parent.withVersion(result.version());
-                        }
+                    ModelResolver.ModelResolverRequest req = new ModelResolver.ModelResolverRequest(
+                            request.getSession(),
+                            null,
+                            repositories,
+                            groupId,
+                            artifactId,
+                            version,
+                            classifier,
+                            extension != null ? extension : "pom");
+                    ModelResolver.ModelResolverResult result = modelResolver.resolveModel(req);
+                    modelSource = result.source();
+                    if (result.version() != null) {
+                        parent = parent.withVersion(result.version());
                     }
                 }
             } catch (ModelResolverException e) {