[MNG-7793] Add generated-source metadata to Maven 4 APIs

[goutamadwant]

Fixes #9196

Summary

Add explicit generated-source metadata to Maven 4's source model and APIs so plugins can distinguish generated files without relying on directory naming conventions.

Implementation

• Add the generated attribute to org.apache.maven.api.model.Source.
• Expose generated state through SourceRoot.
• Add explicit generated-source registration through ProjectManager.
• Preserve the value through model XML, consumer POM, and legacy resource adapter paths.
• Keep existing and legacy source registration defaulting to non-generated.
• Preserve first-registration-wins behavior for duplicate source roots.

Verification

• mvn --batch-mode verify
• Focused API, implementation, and core module verification
• XML round-trip and consumer POM coverage
• Generated registration, duplicate registration, and legacy resource mutation coverage

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• Unit tests cover the behavioral changes.
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[gnodet]

We need to target Maven 4.1.0, hence the model version 4.2.0.

[goutamadwant]

Thanks @gnodet. Updated the generated field to target model version 4.2.0 and moved the corresponding XML round-trip and consumer POM coverage to the 4.2.0 namespace. Let me know if this looks good now.

[slachiewicz]

One more thing to address, please rebase and squash commits

[goutamadwant]

@slachiewicz Rebased onto the latest master and squashed the changes into a single signed commit. Thanks for the review.!