fix(admin-token-issuer-proxy): separate readiness from liveness

[mikeyrcamp]

What

• use the proxy's dependency-aware /readyz endpoint for Kubernetes readiness
• keep /healthz as the process liveness endpoint

Why

The admin token issuer proxy can start before API Keys has completed a cold
start. A compatible application image keeps the process live while retrying
transient metadata failures, but it must not receive traffic until that
metadata is initialized.

Related to #1229.

Validation

• focused helm template assertion confirms readiness renders /readyz
• helm lint deploy/helm/admin-token-issuer-proxy/chart ...
• tools/ci/check-helm-charts (14 charts linted and rendered)

Dependency

This PR must remain draft until a compatible application image containing the
/readyz endpoint is released and the default image tag is updated. The
application change is prepared, but its owning source repository is currently
archived.

No third-party dependencies are added.

[coderabbitai]

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