Fix compile cache artifact escalation

[tfactor2]

Summary

Fix compile-only cache entries so target/classes is cached as an attached output archive instead of being serialized as the Maven project's main artifact.

This prevents a mvn compile cache hit from poisoning later mvn package/mvn install runs with a directory-backed main artifact while keeping compile output caching enabled for jar modules. The change also keeps a compatibility guard for legacy cache entries that already contain a directory-backed main artifact.

A zipped standalone reproducer is included under src/test/projects/maven-cache-install-after-compile-repro.zip.

Tests

• mvn test
• Targeted smoke ITs for compile -> package and compile -> install
• Standalone reproducer against the built snapshot
  maven-cache-install-after-compile-repro.zip

[Copilot]

Pull request overview

This PR fixes a lifecycle phase escalation bug where a mvn compile cache hit could “poison” subsequent mvn package/mvn install runs by restoring target/classes as the project’s main artifact. The change ensures compile outputs are cached/restored as attached output archives instead of being serialized as the main Maven artifact, and adds a compatibility guard for legacy cache entries that stored a directory-backed main artifact.

Changes:

• Cache compile outputs (target/classes, target/test-classes) as attached outputs (zipped) rather than as the main project artifact.
• Add restoration-time guards/reset logic to prevent legacy directory-backed main artifacts from satisfying package-or-later requests.
• Add/extend smoke ITs to assert buildinfo contents for compile -> package and compile -> install escalation scenarios.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/org/apache/maven/buildcache/CacheControllerImpl.java	Stops treating directory-backed artifacts as main artifacts for compile-only cache; attaches compile outputs; adjusts save/restore/staging behavior accordingly.
src/main/java/org/apache/maven/buildcache/BuildCacheMojosExecutionStrategy.java	Adds legacy-cache guarding and artifact reset logic when escalating to package or later phases.
src/test/java/org/apache/maven/buildcache/its/lifecyclephases/CompileThenPackageEscalationTest.java	Strengthens IT by asserting buildinfo.xml does not store a directory-backed main artifact and that package-level entries point to the JAR.
src/test/java/org/apache/maven/buildcache/its/lifecyclephases/CompileThenInstallEscalationTest.java	New IT covering compile -> install escalation with the same buildinfo assertions.

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[tfactor2]

fixed

[tfactor2]

[explanation] #Why saveDirectoryArtifact(...) is removed:

The method made a directory-backed Maven project artifact saveable by zipping it and storing it through the main artifact path. That preserved the invalid metadata shape that caused #449: target/classes is a compile output directory, not the project's packaged main artifact.

With this change, saveProjectArtifact(...) only saves regular files produced by package-or-later mojos. Compile outputs are still cached, but through attachCompileOutputs(...) / attached output archives, so jar modules keep compile-cache benefits without letting target/classes poison later package or install executions.

[cstamas]

Please rebase against latest master.