feat: Add model auto-switch functionality for improved error handling

[freezed-corpse-143]

PR introduces automatic model switching capability to enhance the reliability and user experience of the opencode system. When a model encounters specific types of errors, the system will automatically switch to an alternative model, with opencode/zen as the final fallback.
Key Features

• Automatic Model Fallback: When a model fails due to authentication errors or non-retryable API errors, the system automatically selects an alternative model from available providers
• Intelligent Error Handling: The system distinguishes between different error types:
  • ✅ Triggers fallback: AuthError, non-retryable APIError
  • ✅ Does not trigger fallback: ContextOverflowError, retryable APIError
• Graceful Degradation: If no alternative providers are available, the system falls back to the free opencode/zen model
• User Notification: TUI displays informative notifications when model switching occurs
• Comprehensive Testing: Full test coverage for both core functions and error scenarios
  Implementation Details

1. src/provider/provider.ts: Added getFallbackModel() function that:
   • Excludes the failed model's provider from alternatives
   • Selects the first available alternative provider
   • Chooses the best model from that provider using existing sorting logic
   • Falls back to opencode/zen when no alternatives exist
2. src/session/retry.ts: Added shouldFallbackToAlternativeModel() function that:
   • Evaluates error types to determine if fallback should be triggered
   • Returns true for AuthError and non-retryable APIError
   • Returns false for ContextOverflowError and retryable errors
3. src/session/processor.ts: Integrated fallback logic into the session processor:
   • Checks for fallback eligibility after retry logic
   • Updates message metadata with the new model
   • Publishes ModelFallback event for TUI notifications
   • Continues processing with the fallback model
4. src/session/index.ts: Added ModelFallback bus event definition with:
   • sessionID: The affected session
   • fromModel: Original model that failed
   • toModel: New model being used
   • reason: Error message that triggered the fallback
5. Test Coverage: Added comprehensive tests in:
   • test/provider/provider.test.ts: Tests getFallbackModel with and without alternatives
   • test/session/retry.test.ts: Tests shouldFallbackToAlternativeModel with all error scenarios
     Usage
     The feature is enabled by default and requires no configuration changes. Users will see TUI notifications when model switching occurs, providing transparency about the automatic recovery process.
     Backward Compatibility

• Fully backward compatible with existing sessions and configurations
• No breaking changes to existing APIs
• Integrates seamlessly with current error handling mechanisms
  Performance Impact
• Minimal performance overhead (fallback logic only executes on errors)
• Debug logs only output at DEBUG level
• No impact on normal operation flow
  Fixes #model-auto-switch-feature

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

PR #13189: feat: add model fallback support with TTFT-based timeout
#13189

This PR appears related because it also implements model fallback functionality, though it uses a different approach (TTFT-based timeout vs. error-triggered auto-switching). You may want to review it to ensure there's no overlap or conflict between the two implementations, and to understand if the existing fallback mechanism should be enhanced or replaced by the auto-switch feature in PR #18383.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.