[Bug]: Infinite retry loop on consistent provider failure

[niStee]

Description

The fallback system can spin in an infinite retry loop when a provider consistently fails. We should cap retry attempts (e.g. at 5) so the fallback system can proceed to the next configured model instead of spinning indefinitely.

Steps to Reproduce

1. Configure a provider that consistently fails (e.g., due to network error or repeated 500 responses).
2. Trigger the fallback system to use this provider.
3. Observe the session logs showing an infinite retry loop without proceeding to the next configured model.

Expected Behavior

The retry loop should be capped at a reasonable limit (e.g., 5 attempts). Once the limit is reached, it should return an exhaustion status so the system can proceed to the next fallback model.

Environment

• OS: Any
• OpenCode version: dev
• Provider/model: Any consistently failing provider

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [Bug]: Session processor retries indefinitely with unbounded exponential backoff — no max retries or circuit breaker #17648: Session processor retries indefinitely with unbounded exponential backoff — describes the same infinite retry loop with no max retry count or circuit breaker, with detailed root cause analysis and proposed fix