dsh-llm-pi-ai-headers: Provider-Based Request Header Configuration and Multi-Key Rotation

An official adapter plugin for DSH Headless, designed to address scenarios such as request header filtering, multi-key rotation, and retry strategies. The plugin uses a standard installation method, preserves the official adapter, and only injects request headers, rotates API keys, and restores the `User-Agent` header, ensuring core features such as reasoning, streaming, and tool calling remain fully intact. Users can configure custom request headers per provider in settings, with support for key rotation (primary key + additional keys) and retry policies. The plugin includes automatic quota exhaustion detection and pool restoration: when the API returns quota-related errors such as 429, it automatically removes exhausted keys and returns them to the pool once they recover.