[GH-11772] Fail-fast consumer POM validation for non-4.0.0 model versions

[gnodet]

Summary

Fixes #11772

When a POM-packaged project (parent POM) uses Maven 4.1.0 features like profile conditions that cannot be stripped during consumer POM transformation, the build now fails fast with actionable guidance instead of silently deploying a consumer POM that Maven 3 and Gradle cannot resolve.

Changes

• Consumer POM validation (DefaultConsumerPomBuilder): After building the consumer POM for a POM-packaged project on the non-flatten path, validates that the computed model version is 4.0.0. If not, throws a MavenException listing the available options:

  1. Enable flattening (maven.consumer.pom.flatten=true)
  2. Preserve the model version (preserve.model.version=true)
  3. Remove the incompatible features

• Managed dependency removal flag (Constants / Features): Added maven.consumer.pom.removeUnusedManagedDependencies property (default: true) to control whether unused managed dependencies are removed during consumer POM flattening.

Scope

The validation only applies to:

• POM-packaged projects (parent POMs) — non-POM projects (jar, war, etc.) are consumed as dependencies where Maven 3 ignores unknown elements, so a higher model version is acceptable (only a warning is logged)
• The non-flatten path — when flattening is enabled, the transformation handles what it can (e.g. mixins are intentionally kept for POM-packaged projects with model version 4.2.0)
• Projects without preserve.model.version=true

Use cases

Scenario	Outcome
4.1.0 parent with root/subprojects only (e.g. JLine)	Consumer POM is naturally 4.0.0 after transformation — no intervention needed
4.1.0 parent with profile conditions	Build fails with actionable error message
4.2.0 POM with mixins + flatten	Existing behavior preserved — mixins kept, model version 4.2.0
4.2.0 POM with mixins, no flatten	Existing mixin-specific error preserved

Test plan

• Unit test: testParentWithConditionsFailsConsumerPom — validates fail-fast for profile conditions
• Unit test: testMultiModuleConsumerPreserveModelVersion — validates preserveModelVersion bypass
• IT: MavenITgh11772ConsumerPom410Test — verifies consumer POMs are 4.0.0 and build POMs retain 4.1.0 content for a parent+child project using 4.1.0 namespace