Refactor MNG-11133 test to use maven-it-plugin-expression

[gnodet]

Summary

• Replace help:effective-pom + Files.readString() + String.contains() with the conventional maven-it-plugin-expression approach using verifier.loadProperties() in MavenITmng11133MixinsPrecedenceTest
• This is the same pattern applied in PR [MNG-8432] Integration test for MNG-8432 using mixins #12461 for MavenITmng8432MixinsPropertiesTest — more robust against whitespace/formatting changes in effective POM XML
• Added the expression plugin to the test resource pom.xml to evaluate project/properties at the validate phase

Test plan

• MavenITmng11133MixinsPrecedenceTest passes locally (1 test, 0 failures)
• CI build passes

🤖 Generated with Claude Code

[gnodet]

AI Review — PR #12502

Verdict: ✅ LGTM (own PR, posting as COMMENT)

Clean refactoring from help:effective-pom string matching to the conventional maven-it-plugin-expression + loadProperties() + assertEquals() approach. Same pattern used by 30+ tests in core-it-suite and applied in PR #12461 for MNG-8432.

What I Appreciate

• Stronger assertion — assertEquals("21", ...) checks the exact resolved property value rather than substring-matching raw XML, making the test both more precise and more resilient to formatting changes.
• Self-documenting failure message — "Property from mixin should override the parent's value" immediately tells a developer what went wrong.
• Clean import hygiene — removed Files and assertTrue (no longer needed), added Properties and assertEquals, no dead imports.

Findings

None — textbook refactoring. Same behavior, better implementation. ✨

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Claude Code on behalf of Guillaume Nodet