docs: require JUnit 5 for new tests in AGENTS.md

[FrankChen021]

Description

Update AGENTS.md to require JUnit 5 (org.junit.jupiter) for new test cases and tell coding agents not to add new tests using JUnit 4 (org.junit).

Motivation

This follows review feedback on PR #19826 asking that new tests use JUnit 5 and the follow-up discussion proposing that the rule be documented in AGENTS.md.

The repository-wide migration is tracked by #13948: JUnit4 to JUnit5 migration, which proposes migrating incrementally while retaining JUnit Vintage for backward compatibility. Completing that migration may take time because legacy JUnit 4 tests remain across many modules. In the meantime, this guidance prevents new tests from adding to the remaining migration work. It does not affect Druid runtime behavior.

Verification

• Confirmed that JUnit 5 is configured across Druid modules.
• Self-reviewed the complete diff against the target branch.
• Ran git diff --check.
• No runtime tests were run because this change only updates agent guidance.

This PR has:

• been self-reviewed.
• added documentation for a repository testing requirement.

[FrankChen021]

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Reviewed 1 of 1 changed files.

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This is an automated review by Codex GPT-5.6-Sol