Make aggregation thread-safe (round 3: use RepositorySystemSession)

[jozic]

Motivation

It happens that maven can load different modules using different classloaders and that MavenSession's data is also not guaranteed to be the same across all modules. The solution is to use RepositorySystemSession to sync across all modules.

For compatibility with Maven < 3.9, uses older get/set api

Checklist

• Add/Update integration tests in src/it
• Add/Update documentation in README

[Copilot]

Pull request overview

This PR addresses thread-safety issues in aggregated coverage report generation by switching from MavenSession to RepositorySystemSession for build-global storage. This ensures proper synchronization across modules that may be loaded with different classloaders.

Changes:

• Replaced MavenSession with RepositorySystemSession for coordinating aggregation across modules
• Updated storage implementation to use backward-compatible get/set API instead of computeIfAbsent for Maven 3.6.3+ compatibility
• Added cleanup logic to remove data from session after aggregation is claimed

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/org/scoverage/plugin/SCoverageReportMojo.java	Injected RepositorySystemSession parameter and updated aggregation coordinator call to use it
src/main/java/org/scoverage/plugin/SCoverageAggregationCoordinator.java	Refactored to use RepositorySystemSession.getData() with backward-compatible synchronization approach
src/it/test_reactor_test_scoped_dependency/pom.xml	Updated description to remove reference to specific scenario name

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