[9.3] (backport #18919) Improve xpack integration test UX

[mergify]

Summary

Three improvements to the Gradle xpack integration test workflow:

1. Make ES and Filebeat extraction incremental. copyEs and copyFilebeat now declare inputs / outputs so Gradle skips re-extraction when the tar.gz is unchanged.
2. Support running a single xpack spec via -PrubyIntegrationSpecs= instead of the full qa/integration suite.

How to test this PR locally

• Run ./gradlew :logstash-xpack:rubyIntegrationTests -PrubyIntegrationSpecs=qa/integration/monitoring/monitoring_is_disabled_spec.rb. Only that spec runs.
• Run the second time ./gradlew :logstash-xpack:rubyIntegrationTests -PrubyIntegrationSpecs=qa/integration/monitoring/monitoring_is_disabled_spec.rb. ES is NOT re-downloaded on second run.
• Run ./gradlew :logstash-xpack:rubyIntegrationTests. All specs run. ES and Filebeat extracted correctly.

  ---

  This is an automatic backport of pull request Improve xpack integration test UX #18919 done by Mergify.

[mergify]

Cherry-pick of fcfcea0 has failed:

On branch mergify/bp/9.3/pr-18919
Your branch is up to date with 'origin/9.3'.

You are currently cherry-picking commit fcfcea0c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   build.gradle
	modified:   x-pack/build.gradle
	modified:   x-pack/src/test/java/org/logstash/xpack/test/RSpecIntegrationTests.java

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   x-pack/AGENTS.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: daae62f

Failed CI Steps

• 🥼 Integration Tests - FIPS mode / part 2-of-6

cc @kaisecheng

[mergify]

This pull request has not been merged yet. Could you please review and merge it @kaisecheng? 🙏