feat: Add configurable max retry limit for SessionRetry mechanism #26675

Problem

When a provider returns persistent 5xx errors (e.g., 500 Internal Server Error), the built-in SessionRetry mechanism retries indefinitely with exponential backoff. This can lead to extremely long wait times (hours) without any way to configure a maximum retry count.

Observed Behavior

When DeepSeek-V4-Pro returns a 500 error, OpenCode retries with exponential backoff:

• Attempt 1: 2s delay
• Attempt 5: 32s delay
• Attempt 10: ~17min delay
• Attempt 13: ~2h 11min delay
• ...continues indefinitely

The retry mechanism has:

• RETRY_INITIAL_DELAY = 2000ms
• RETRY_BACKOFF_FACTOR = 2
• RETRY_MAX_DELAY = 2147483647ms (~24.8 days)
• No maximum retry count limit

Code Reference

The retry policy in SessionProcessor uses Effect's n.retry(cr.policy({...})) which has no configurable max attempts. The maxRetries: d.retries ?? 0 only applies to the Vercel AI SDK layer, not the Effect retry policy.

Proposed Solution

Add a configurable maximum retry limit to the SessionRetry mechanism:

// opencode.json
{
  "session": {
    "retry": {
      "maxAttempts": 5,  // Stop retrying after 5 attempts
      "maxDelay": 30000  // Optional: cap max delay at 30s
    }
  }
}

Or per-agent configuration:

{
  "agent": {
    "build": {
      "retry": {
        "maxAttempts": 5
      }
    }
  }
}

Use Cases

1. Provider outages: When a provider is down for extended periods, users should be able to set a limit and let fallback mechanisms (like model switching plugins) take over
2. Cost control: Prevent extremely long retry loops that consume time/resources
3. User experience: Users can set reasonable limits and manually intervene when needed

Current Workaround

None. Users must manually abort sessions when retries become excessive. Plugin-based fallback systems (like oh-my-openagent) can switch models but cannot control OpenCode's internal retry count.

Environment

• OpenCode version: latest (binary)
• Platform: Linux
• Provider: DeepSeek-V4-Pro (direct API)