- Updated the gemini-image test harness to use CustomToolContext, ModelRegistry, and ReadonlySessionManager types.
- Expanded getHeaderValue in gemini-image tests to handle string-array header values.
- Adjusted the resolve.rs stale-selector test to use a let-else Err(err) binding.
- Clarified the fence_marker doc comment to describe CommonMark triple-marker detection semantics.
- Added designer model role for UI/UX design tasks with Gemini 3.1 Pro as default model.
- Implemented model role fallback list support enabling automatic fallback to next available model when primary is unavailable.
- Refactored model role resolution to support multiple fallback patterns per role with thinking level mapping.
- Updated designer agent to use pi/designer role alias instead of explicit model list, removing spawns configuration.
- Added test coverage for model resolver fallback patterns and designer role override preferences.
- Added test coverage for geminiImageTool X-Title header routing through OpenRouter.
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