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/ai_endpoint_mgr/CMakeLists.txt

@@ -0,0 +1,32 @@
+# ============================================================
+# AI endpoint manager plugin / AI endpoint manager 插件
+# ============================================================
+
+find_package(Boost REQUIRED CONFIG)
+
+add_library(plugin_ai_endpoint_mgr SHARED
+    src/endpoint_mgr_plugin.cpp
+)
+
+target_include_directories(plugin_ai_endpoint_mgr
+    PRIVATE
+        ${CMAKE_SOURCE_DIR}/dstalk_core/include
+        ${CMAKE_SOURCE_DIR}/plugins_upper/ai_common/include
+)
+
+target_link_libraries(plugin_ai_endpoint_mgr
+    PRIVATE
+        dstalk
+        ai_common
+        dstalk_boost_config
+        boost::boost
+)
+
+# cxx_std_20 已由 dstalk 和 ai_common (PUBLIC) 传播，无需重复声明
+# cxx_std_20 is already propagated by dstalk and ai_common (PUBLIC); no need to redeclare
+
+set_target_properties(plugin_ai_endpoint_mgr PROPERTIES
+    PREFIX ""
+    LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"
+    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"
+)