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/include/dstalk/dstalk_host.h

@@ -65,6 +65,7 @@ typedef struct {
     /* --- 服务注册表 / service registry --- */
     int  (*register_service)(const char* name, int version, void* vtable);
     void*(*query_service)(const char* name, int min_version);
+    void (*unregister_service)(const char* name);
 
     /* --- 事件总线 / event bus --- */
     int  (*event_subscribe)(int event_type, dstalk_event_handler_fn handler, void* userdata);

dstalk_core/include/dstalk/dstalk_services.h

@@ -15,7 +15,7 @@ extern "C" {
 #endif
 
 /* ---- AI 服务 vtable / AI service vtable ---- */
-/* 以名称如 "ai.openai" 或 "ai.anthropic" 注册 / Registered under names such as "ai.openai" or "ai.anthropic" */
+/* 以名称如 "ai_openai" 或 "ai_anthropic" 注册 / Registered under names such as "ai_openai" or "ai_anthropic" */
 typedef struct {
     /* 配置服务商连接 (base_url, api_key, model 等) / Configure provider connection (base_url, api_key, model, etc.) */
     int (*configure)(const char* provider, const char* base_url,