dsh-draw-router: Add unified drawing routing to DeepSeek Harness

`dsh-draw-router` is a unified drawing routing plugin for DeepSeek Harness (DSH), designed to address the inconsistencies of interfaces across different OpenAI-compatible image generation services. By registering the two Agent tools `draw_image` and `draw_list_sources`, the plugin allows DSH Agents to directly invoke drawing capabilities during conversations. It also supports the automatic discovery of available image models without relying on a fixed model list. Compatible with multiple providers such as SenseNova, StepFun, Qwen, and Hunyuan, it only requires specifying the API Root during configuration to automatically adapt paths. In addition to Agent integration, the plugin provides a REST API for managing drawing sources and initiating drawing requests.