feat: Add LSP plugin unit tests and frontend common initialization library

- Introduced `dstalk_lsp_plugin_test` for testing LSP plugin functionalities including `lsp_trim`, `lsp_frame_message`, and `lsp_parse_content_length`.
- Created `dstalk_frontend_common` static library to encapsulate shared initialization logic for frontend components (CLI, GUI, Web).
- Implemented configuration file discovery and service querying in `dstalk_frontend_init`.
- Added internal headers for LSP and Anthropic plugins to facilitate unit testing.
- Established a mailroom system for asynchronous message passing between stateless agents, enhancing coordination and context management.

CMakeLists.txt

@@ -12,6 +12,7 @@ option(DSTALK_BUILD_WEB "Build the web UI frontend" OFF)
 option(DSTALK_BUILD_TESTS "Build dstalk tests" ON)
 
 add_subdirectory(dstalk_core)
+add_subdirectory(dstalk_frontend_common)
 add_subdirectory(dstalk_cli)
 # 插件按依赖层级分三个目录 / Plugins split into three directories by dependency tier
 add_subdirectory(plugins_base)