feat: detect image input support in test-connection endpoint - #496

Summary

• Add image-support detection to POST /api/config/test-connection: after the connectivity test passes, the endpoint determines whether the model accepts image input.
• Two-tier strategy: first check the built-in provider catalog (covers exact match, aliases, cross-provider lookup, and vendor-prefix stripping for proxy providers like OpenRouter); if no catalog entry is found, send a live probe with a minimal 1x1 PNG to the provider.
• Response now includes supportsImage (true / false / null) and imageCheckSource ("catalog" or "probe") when connectivity succeeds. Failure responses are unchanged.

Test plan

• Catalog hit with vision model returns supportsImage: true, imageCheckSource: 'catalog'
• Catalog hit with text-only model returns supportsImage: false, imageCheckSource: 'catalog'
• Cross-provider catalog lookup (e.g. OpenRouter → Anthropic) returns correct result
• Catalog miss + probe success returns supportsImage: true, imageCheckSource: 'probe'
• Catalog miss + probe 4xx returns supportsImage: false, imageCheckSource: 'probe'
• Catalog miss + probe timeout returns supportsImage: null, imageCheckSource: 'probe'
• Connectivity failure does not include supportsImage field
• All 32 existing + new tests pass

Made with Cursor