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>

tests/smoke_test.cpp

@@ -186,7 +186,7 @@ int main()
     // 测试服务查询: ai（可能因为没有真实 API key 而失败，但服务应存在）
     // Test service query: ai (may fail without real API key, but service should exist)
     const char* ai_provider = dstalk_config_get("ai.provider");
-    if (!ai_provider) ai_provider = "ai.openai";
+    if (!ai_provider) ai_provider = "ai_openai";
     auto* ai = static_cast<const dstalk_ai_service_t*>(
         dstalk_service_query(ai_provider, 1));
     if (ai) {
@@ -598,12 +598,12 @@ int main()
                 std::cout << "[OK] R3: http error path, no response body (connection refused)\n";
             }
         } else {
-            // 回退：测 AI 服务 (ai.openai) 错误路径
-            // Fallback: test AI service (ai.openai) error path
+            // 回退：测 AI 服务 (ai_openai) 错误路径
+            // Fallback: test AI service (ai_openai) error path
             auto* ai_svc = static_cast<const dstalk_ai_service_t*>(
-                dstalk_service_query("ai.openai", 1));
+                dstalk_service_query("ai_openai", 1));
             if (ai_svc) {
-                std::cout << "[OK] R3: ai.openai service found (http fallback)\n";
+                std::cout << "[OK] R3: ai_openai service found (http fallback)\n";
                 dstalk_message_t msg = {"user", "hi", nullptr, nullptr};
                 dstalk_chat_result_t r = ai_svc->chat(&msg, 1, "", nullptr);
                 // api_key="test-key" 为无效 key，应返回 error result 而非崩溃