PROJQUAY-10873: fix(mirror): prevent pod loss during securityContext override

[Marcusk19]

Summary

• Root cause fix: GetReplicasOverrideForComponent returned nil when a component had overrides (e.g. securityContext) but no explicit replicas setting. This caused the middleware to omit the replicas field from server-side apply, releasing operator ownership and letting Kubernetes revert desired replicas from 2→1.
• Fix: Return the default of 2 replicas consistently, regardless of whether other overrides are present, matching the behavior when Overrides == nil.
• Defense-in-depth: Keeps the existing maxUnavailable=0 strategy from the previous commit to prevent pod loss during any rollout where new pods fail admission.

Test plan

• Added TestGetReplicasOverrideForComponent covering: no overrides (default 2), overrides with securityContext but no replicas (default 2), explicit replicas override, and component not found
• All existing tests pass
• QE verification: apply securityContext override on mirror component without replicas → mirror pods should remain at 2

[openshift-ci]

@Marcusk19: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/ocp-latest-e2e	09a5237	link	true	/test ocp-latest-e2e

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Details

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[openshift-ci]

PR needs rebase.

Details

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