raft: return error when ConfChange is rejected due to pending change

[SherlockShemol]

Previously, when a ConfChange proposal was rejected (e.g., due to an existing pending config change), the proposal was silently converted to a no-op entry without returning any error. This silent failure could lead to security issues where administrators believe a configuration change succeeded when it was actually ignored.

This commit adds error notification while preserving the original behavior of appending a no-op entry for log consistency:

1. When a ConfChange is rejected, it is still converted to a no-op (maintaining backward compatibility and log consistency)
2. BUT now returns ErrProposalDropped to notify the caller

This allows callers to:

• Detect when their ConfChange was rejected
• Take appropriate action (e.g., wait and retry)
• Log warnings or alerts

The fix is conservative: it maintains full backward compatibility with existing log structures while adding explicit error notification.

#354

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: SherlockShemol
Once this PR has been reviewed and has the lgtm label, please assign serathius for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @SherlockShemol. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.