feat: add AI endpoint manager plugin with configuration and routing capabilities

- Introduced `ai_endpoint_mgr` plugin to manage multiple AI provider endpoints.
- Added configuration reference documentation for `config.toml`.
- Implemented endpoint loading, active endpoint switching, and model mutation.
- Included error handling for missing endpoints and configuration failures.
- Developed unit tests covering various scenarios including error paths and concurrency.

plugins_upper/CMakeLists.txt

@@ -6,3 +6,4 @@ add_subdirectory(ai_common)    # 共享 AI 工具库（静态库）/ shared AI u
 add_subdirectory(context)      # 依赖 session / depends on session
 add_subdirectory(openai)       # 依赖 http, config, ai_common / depends on http, config, ai_common
 add_subdirectory(anthropic)    # 依赖 http, config, ai_common / depends on http, config, ai_common
+add_subdirectory(ai_endpoint_mgr) # 路由多个 AI endpoint / routes multiple AI endpoints