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.

dstalk_frontend_common/CMakeLists.txt

@@ -0,0 +1,17 @@
+# ============================================================
+# dstalk_frontend_common — 前端公共初始化静态库
+# ============================================================
+
+add_library(dstalk_frontend_common STATIC
+    src/frontend_common.cpp
+)
+
+target_include_directories(dstalk_frontend_common
+    PUBLIC include
+)
+
+target_link_libraries(dstalk_frontend_common
+    PUBLIC dstalk
+)
+
+target_compile_features(dstalk_frontend_common PUBLIC cxx_std_20)