Fix flaky P2 test failures caused by parallel fork lock contention

[mirkoalicastro]

This PR aims to fix Fix flaky P2 test failures.

Context

lib-extra:test runs with maxParallelForks = Runtime.runtime.availableProcessors().intdiv(2).

Each Gradle test fork is a separate JVM. When multiple forks simultaneously call model.query(), they race for a process-level file lock causing one of the 2 forks to get an IllegalStateException (P2 operation already in progress).

TestP2Provisioner already has sync mechanism, but that only works within one JVM and not across multiple forks.

Fix

Override maxParallelForks = 1 in lib-extra/build.gradle. One fork is enough because the TestP2Provisioner synch handles thread safety within one JVM, and its disk cache ensures P2 deps are provisioned once and reused across all test classes.

I don't expect meaningful slowdown since with 2 forks the parallel gain was already lost to lock failures and testRetry reschedules. Also, I expect this to make the pipeline more stable.