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.

plugins_base/lsp/src/lsp_plugin.cpp

@@ -12,6 +12,7 @@
 
 #include "dstalk/dstalk_host.h"
 #include "dstalk/dstalk_services.h"
+#include "lsp_internal.hpp"
 
 #include <boost/json.hpp>
 #include <boost/json/src.hpp>
@@ -311,7 +312,7 @@ static LspState g_lsp;
 // ============================================================================
 
 // 去除 string_view 首尾空白 / Trim leading and trailing whitespace from a string_view.
-static std::string_view trim(std::string_view sv) {
+std::string_view lsp_trim(std::string_view sv) {
     while (!sv.empty() && (sv.front() == ' ' || sv.front() == '\t' ||
                            sv.front() == '\r' || sv.front() == '\n'))
         sv.remove_prefix(1);
@@ -322,7 +323,7 @@ static std::string_view trim(std::string_view sv) {
 }
 
 // 将 JSON-RPC 消息体包装在 LSP 头中 (Content-Length: ...\r\n\r\n) / Wrap a JSON-RPC message body in an LSP header (Content-Length: ...\r\n\r\n).
-static std::string frame_message(const std::string& body) {
+std::string lsp_frame_message(const std::string& body) {
     std::string frame;
     frame.reserve(64 + body.size());
     frame += "Content-Length: ";
@@ -333,8 +334,8 @@ static std::string frame_message(const std::string& body) {
 }
 
 // 从 LSP 头行中解析 Content-Length 值。解析失败返回 -1 / Parse the Content-Length value from an LSP header line. Returns -1 on parse failure.
-static int parse_content_length(const std::string& line) {
-    auto sv = trim(std::string_view(line));
+int lsp_parse_content_length(const std::string& line) {
+    auto sv = lsp_trim(std::string_view(line));
     const char prefix[] = "Content-Length:";
     const size_t prefix_len = sizeof(prefix) - 1;
 
@@ -368,7 +369,7 @@ static int send_request(const std::string& method, const json::object& params) {
     msg["params"] = params;
 
     std::string body = json::serialize(msg);
-    g_lsp.proc.write(frame_message(body));
+    g_lsp.proc.write(lsp_frame_message(body));
     return id;
 }
 
@@ -380,7 +381,7 @@ static void send_notification(const std::string& method, const json::object& par
     msg["params"] = params;
 
     std::string body = json::serialize(msg);
-    g_lsp.proc.write(frame_message(body));
+    g_lsp.proc.write(lsp_frame_message(body));
 }
 
 // ============================================================================
@@ -457,11 +458,11 @@ static void reader_loop() {
                 }
 
                 // header 块以空行结束 / header block ends with empty line
-                auto sv = trim(std::string_view(line));
+                auto sv = lsp_trim(std::string_view(line));
                 if (sv.empty()) break;
 
                 // 累积 Content-Length；遇到其他 header 不丢弃，继续读取下一行 / Accumulate Content-Length; don't discard other headers, continue reading next line
-                int len = parse_content_length(line);
+                int len = lsp_parse_content_length(line);
                 if (len >= 0) content_length = len;
             }