docs: clarify Gradle test task structure in AGENTS.md

[andrewvc]

Summary

• Document that --tests filtering must be used on javaTests or rubyTests directly, not the top-level test task (which excludes all classes and delegates)
• Explain that some Java test classes (e.g. OutputDelegatorTest, ConfigCompilerTest) live in rubyTests due to Ruby infrastructure dependencies
• Add example command for running a single test from rubyTests

Test plan

• Verify AGENTS.md renders correctly on GitHub

🤖 Generated with Claude Code

[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.

[mergify]

This pull request does not have a backport label. Could you fix it @andrewvc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[mergify]

This pull request does not have a backport label. Could you fix it @andrewvc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label