♻️ refactor(lsp_server): move entrypoints out of named modules

- export Run() from launcher module and add non-module main TU
- apply same pattern to module-based test executables
- migrate utils/string implementation fully into string.cppm and drop string.cpp
- point clangd to linux build compilation database

lsp-server/test/test_scheduler/CMakeLists.txt

@@ -15,6 +15,7 @@ if(UNIX AND NOT APPLE)
 endif()
 
 set(SOURCES
+    main.cc
     test_async_executor.cppm)
 
 add_executable(${PROJECT_NAME} ${SOURCES})