W17: extract ai_common shared module + fix anthropic data race + brace bugs

- New plugins_upper/ai_common/ static library: shared PluginConfig, ToolCallAccum,
  StreamContext, secure_zero, extract_host_port, serialize_tool_calls, free_chat_result
- Refactored openai/anthropic plugins to use dstalk_ai:: namespace from ai_common
- Fixed anthropic g_config raw pointer → std::atomic (data race)
- Added SSE parse error counter with threshold abort (kMaxSseParseErrors=5)
- Fixed missing closing brace in both plugins' error-body catch block
- Updated test targets: ai_common include path + link, using namespace dstalk_ai
- plugin_loader_test: added stub_unreg + service_registry.cpp for unregister_service
- Includes pre-existing uncommitted changes from prior waves

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dstalk_core/src/service_registry.hpp

@@ -10,6 +10,7 @@
 #include <shared_mutex>
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 namespace dstalk {
 
@@ -32,6 +33,12 @@ public:
     // 注销服务 / Unregister a named service
     void unregister_service(const char* name);
 
+    // 列出所有已注册服务名称（用于 diff/遍历）/ List all currently registered service names (for diff / iteration)
+    std::vector<std::string> list_service_names() const;
+
+    // 清空所有注册服务 / Remove all registered services
+    void clear();
+
 private:
     struct ServiceEntry {
         std::string name;