dsh-model-router: Add virtual model providers to DSH and route requests based on active providers.

`dsh-model-router` is a community plugin for DSH designed to address management pain points in multi-provider and multi-model configurations. By registering a virtual provider named `model-router`, it aggregates models scattered across various providers into a single selector, supporting strict merging based on model IDs. Users can view the available providers for each model in the model selection menu or settings card, and support one-click switching of the currently active provider as well as drag-and-drop adjustment of priority order. The plugin adopts a real-time routing mechanism, preserving the virtual provider identifier within the session and resolving/delegating to the currently active real provider only when an LLM request is issued, ensuring consistent routing logic for main agents, sub-agents, and tool calls. The plugin features automatic synchronization capabilities, allowing it to automatically clean up configurations and fall back to active states when new providers or models are added or removed. It is suitable for Web, C...