MCP: Image refs (#1987)
* Add image reference support for MCP tool calls
Introduces image reference resolution and attachment for MCP tool arguments, allowing tools to accept lightweight image reference strings (e.g. 'latest', 'image_1') and receive resolved image payloads. Updates tool invocation and flow logic to use these references, and improves tool prompt instructions for image input handling.
* Enable multiple file uploads in ChatInput
* Refactor multimodal model selection logic
* Remove 'latest' image reference support
Eliminates handling of the 'latest' image reference in image resolver logic and updates related comments and prompts to only support 'image_1', 'image_2', etc. This simplifies image reference usage and clarifies instructions for tool input parameters.
* Refactor imageRefs to fileRefs for tool payloads
