Make ICMS instance-placement arbitration race-safe with NATS

Description

Make ICMS instance-placement arbitration consistent and race-safe by using NATS for request broadcast and a request-scoped JetStream consumer to collect cluster acknowledgements. Preserve the existing auction model while ensuring one ICMS instance owns each decision.

Definition of Done

• Instance-creation requests can target the intended region or set of clusters over NATS.
• Acknowledgements for each request are collected by a short-lived JetStream consumer owned by the publishing ICMS instance.
• Exactly one eligible cluster is selected and every responder receives a deterministic winning or non-winning result.
• No-responder and publisher-failure paths use bounded timeouts and retry-safe behavior.
• The selected result is persisted so retries can resume without repeating a completed arbitration.
• The new path can run without side effects for comparison with the current flow before a feature-flagged cutover.

Out of Scope

Replacing messaging for unrelated components is not part of this issue.