raft: return error for rejected conf change

[NasitSony]

Summary

Return an explicit error when a configuration change proposal is rejected by validation and converted to a no-op entry.

Motivation

Currently, rejected configuration changes are silently converted to EntryNormal entries and Step returns nil. This makes it difficult for callers to detect that a configuration change was rejected and take corrective action.

This change preserves the existing no-op conversion behavior while returning an explicit error to notify the caller.

Testing

• Extended TestStepIgnoreConfig to verify the returned error.
• Updated interaction test expectations.
• Ran go test ./....

Fixes #354

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @NasitSony. Thanks for your PR.

I'm waiting for a etcd-io 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.

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