Drop @Override on MockHelixAdmin.getBatchDisabledInstances for Helix compat

[sjainit]

Summary

org.apache.helix.HelixAdmin.getBatchDisabledInstances(String) is being removed from the Helix HelixAdmin interface in linkedin/helix#253 (retiring the deprecated cluster-level DISABLED_INSTANCES config; disablement now lives at instance-level config).

MockHelixAdmin (test scope) implements HelixAdmin and carries an @Override on this method. Once Ambry bumps to the Helix release that removes it, the annotation will fail to compile:

error: method does not override or implement a method from a supertype

This was flagged during review of the Helix PR — the audit found Ambry's MockHelixAdmin (and one other repo) as external implementers that would break on the next Helix bump.

Change

Drop the @Override annotation while keeping the existing (already unused) stub. This makes the mock compile against both the current Helix version (method still present → the stub satisfies it) and the post-removal version (method gone → the stub is just an unused method), so it can merge independently of the Helix release timing.

The stub can be deleted entirely in a later cleanup once Ambry adopts the Helix release that removes the method.

Impact

• Test scope only, no production code. Behavior unchanged (the stub already threw IllegalStateException and has no callers).
• Verified via a LinkedIn-wide code search that getBatchDisabledInstances has no callers anywhere — it is dead interface surface.

Validation

CI compile against the current Helix version (annotation removal on a mock; no signature or behavior change).

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Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com