fix: handle providers with no available models to prevent crashes

[ahosker]

Summary

• Fix runtime crashes when providers have no available models
• Add validation in provider merge process
• Prevent empty model arrays from breaking ACP agent functionality
• Graceful degradation with error logging for debugging

Fixes

• Prevents crashes in model selection dialogs
• Handles edge cases in provider configuration merges
• Improves error handling in server API endpoints

Test

OpenCode continues to work against the earlier failed models.json (attatched) models_broken.json

Closes #5858

[rekram1-node]

/review

[ahosker]

Committed the type safety fixes.

[rekram1-node]

/review

[github-actions[bot]]

lgtm