Support Foundry embeddings and image generation

## Summary
Extend Foundry support beyond chat/workflow agents into embeddings and image generation where provider capabilities allow it.

## User Value
Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

## Proposed Behavior
Define provider capability discovery, per-endpoint model types, admin test connection, fallback behavior, and migration from Azure OpenAI-specific settings.

## Acceptance Criteria
- [ ] The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
- [ ] Permission, configuration, and existing-feature interactions are accounted for.
- [ ] Tests, documentation, or validation notes are added as appropriate.

## Notes
- Source backlog entry: Foundry For Embeddings And Image Generation
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension
- Current anchors: Foundry agent runtime/docs, model endpoint support, image generation settings
- Proposed roadmap priority: P2
- Proposed roadmap size: L