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_web/src/main.cpp

@@ -507,7 +507,7 @@ int main(int argc, char* argv[])
 
     // 查询插件服务 / Query plugin services
     const char* ai_provider = dstalk_config_get("ai.provider");
-    if (!ai_provider) ai_provider = "ai.openai";
+    if (!ai_provider) ai_provider = "ai_openai";
     g_ai      = static_cast<const dstalk_ai_service_t*>(dstalk_service_query(ai_provider, 1));
     g_session = static_cast<const dstalk_session_service_t*>(dstalk_service_query("session", 1));