[feature] explicit last-resort cooldown policy for failover combos #5691

Area

Proxy and routing

What are you trying to accomplish?

Allow a failover combo to treat a designated last-resort target differently from ordinary targets so a brief cooldown on a preferred target does not immediately route traffic to a target intended only for emergency fallback.

What prevents this today?

The ordinary cooldown/failover selector has no explicit policy that says a target is a last resort. Without that distinction, transient cooldown state can cause routing to a target the operator intended to use only after all normal targets are unavailable.

What should OpenCodex do?

Support an explicit last-resort cooldown policy for combo targets. A last-resort target should remain eligible when normal targets are genuinely unavailable, but the router should be able to wait for a short recoverable cooldown on a preferred normal target instead of prematurely dispatching the last-resort target. Existing default behavior should remain unchanged unless the policy is configured.

Example usage or interface

{
  "strategy": "failover",
  "cooldownWaitPolicy": "before-last-resort",
  "targets": [
    {"provider":"provider-a","model":"model-a"},
    {"provider":"provider-b","model":"model-b"},
    {"provider":"provider-c","model":"model-c","lastResort":true}
  ]
}

Alternatives or workarounds

Operators can manually reorder targets or tune global cooldowns, but those approaches conflate priority with temporary health and can add unnecessary latency or cost. A local policy implementation has been validated without changing default behavior.

Additional context

This is intentionally a routing-policy feature rather than a claim that existing failover ordering is incorrect. It is useful for local/offline or high-latency emergency targets that should remain absolute fallback only.

Checks

• I searched existing issues and documentation.
• This request describes a concrete OpenCodex workflow rather than merely naming a desired technology.
• I removed secrets and personal data.