[Parth Agrawal (pagrawal10)]

Backport of apache#19139.

Use Kubernetes container readiness (in addition to labels/annotations) to decide whether a pod is available for service discovery. Pods whose containers are not ready (e.g. OOM-killed but still carrying announcement labels) are now removed from discovery caches and re-added once ready again.

• DefaultK8sApiClient: filter not-ready pods on list; remap watch events (MODIFIED->ADDED when ready, MODIFIED->NOT_READY when unready, skip not-ready ADDED events).
• WatchResult: add MODIFIED and synthetic NOT_READY event types.
• BaseNodeRoleWatcher: add childRemoved(node, skipIfUnknown) overload to silently ignore removals of nodes not in the cache.
• K8sDruidNodeDiscoveryProvider: treat NOT_READY like DELETED, using skipIfUnknown=true for all k8s discovery removals.
• Docs/spelling updates for readiness probe guidance.

Adapted for the 30.0.1 branch: new unit test converted from JUnit 5 to JUnit 4 to match the module, and JUnit annotations adjusted accordingly.

Fixes #XXXX.

Description

Fixed the bug ...

Renamed the class ...

Added a forbidden-apis entry ...

Release note

For tips about how to write a good release note, see Release notes.

---

Key changed/added classes in this PR

• MyFoo
• OurBar
• TheirBaz

---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.