Add validation for mixins in consumer POM without flattening (fixes #11456)

[gnodet]

Description

This PR adds validation to prevent creating consumer POMs when mixins are present without flattening enabled.

Changes

• Added validation in DefaultConsumerPomBuilder: Checks for mixins when flattening is disabled and throws MavenException with a helpful error message
• Added integration test MavenITgh11456MixinsConsumerPomTest with two scenarios:
  1. testMixinsWithoutFlattening: Verifies build fails with proper error message
  2. testMixinsWithFlattening: Verifies build succeeds and mixins are removed from consumer POM when flattening is enabled
  3. testMixinsWithPreserveModelerVersion: Verifies that mixins are kept when preserving the modelVersion in the consumer POM.
• Created test resources: POMs using mixins with local repository containing mixin artifacts

Problem

Mixins require model version 4.2.0 and cannot be part of the consumer POM. When consumer POM flattening is disabled (maven.consumer.pom.flatten=false), mixins would remain in the consumer POM, which is invalid.

Solution

Maven now fails early with a clear error message that guides users to either:

• Enable flattening by setting maven.consumer.pom.flatten=true, or
• Set the preserve.model.version=true flag on the POM, or
• Remove mixins from their POM

Testing

• Full build passes: mvn clean install -DskipTests
• Code formatting verified: mvn spotless:check
• Integration tests created to verify both failure and success scenarios

Fixes #11456

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Pull Request opened by Augment Code with guidance from the PR author

[cstamas]

Note: I keep coming back to this, but we may tie these features to "maven level" (model version?) instead to have these freely (and independent) flags for ON/OFF them?

[gnodet]

Note: I keep coming back to this, but we may tie these features to "maven level" (model version?) instead to have these freely (and independent) flags for ON/OFF them?

This is purely bulld time, as it's used when unit testing Maven 4 plugins. The flag has been added to avoid having to escape XML-reserved chars in some cases (<, >, ', &...), that's all. It's on by default, so that plugins migrating from 4.0 to 4.1 may have to adapt if they're using such characters in their values.

[gnodet]

Note: I keep coming back to this, but we may tie these features to "maven level" (model version?) instead to have these freely (and independent) flags for ON/OFF them?

This is purely bulld time, as it's used when unit testing Maven 4 plugins. The flag has been added to avoid having to escape XML-reserved chars in some cases (<, >, ', &...), that's all. It's on by default, so that plugins migrating from 4.0 to 4.1 may have to adapt if they're using such characters in their values.

Sorry, I mixed the PRs.
What do you want to tie to maven level ? We disabled flattening by default in rc-5 because the behaviour is less controlable. This is currently incompatible with mixins. Are you suggesting we enable back flattening for 4.1.0 ? @rmannibucau

[gnodet]

Note: I keep coming back to this, but we may tie these features to "maven level" (model version?) instead to have these freely (and independent) flags for ON/OFF them?

This is purely bulld time, as it's used when unit testing Maven 4 plugins. The flag has been added to avoid having to escape XML-reserved chars in some cases (<, >, ', &...), that's all. It's on by default, so that plugins migrating from 4.0 to 4.1 may have to adapt if they're using such characters in their values.

Sorry, I mixed the PRs. What do you want to tie to maven level ? We disabled flattening by default in rc-5 because the behaviour is less controlable. This is currently incompatible with mixins. Are you suggesting we enable back flattening for 4.1.0 ? @rmannibucau

In any case, this can be done in a follow-up PR. The point is that mixins require flattening so that they can be translated into 4.0.0 model.

[gnodet]

FTR, I've slightly modified the check to take into account the preserveModelVersion which can be used to keep the consumer POM with a modelVersion of 4.2.0, in which case we can safely keep mixin elements.