[Feature]: readiness probe for pub/sub

[evacchi]

Problem Statement

#239 (comment)

Readiness depth is asymmetric across backends. Only RedisMQFlow and RedisSortedSetFlow implement pipeline.HealthChecker (via rdb.Ping). For gcp-pubsub / gcp-pubsub-gated, PubSubMQFlow does not implement it, so this type assertion fails and checker stays nil. As a result /readyz for the pub/sub backends only reflects the in-process ready flag and never verifies broker/subscription connectivity — a pod with an unreachable Pub/Sub backend will still report ready.

Not blocking — just flagging so the pub/sub readiness limitation is understood and not mistaken for full backend readiness. Pub/Sub HealthCheck support will be added in a follow-up.

We should address this

Proposed Solution

we should make the readiness probe works correctly and add Pub/Sub HealthCheck support

Alternatives Considered

n/a

Willingness to Contribute

Yes, with guidance

Additional Context

No response