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>

docs/explanation/plugin-lifecycle.md

@@ -87,7 +87,7 @@ Host 收到非零返回值后，会跳过后续插件的初始化并报告警告
 **契约 3：register_service 注册自己的服务。** 插件将自己的 vtable 注册到服务注册表后，其他依赖它的插件才能在后续的 `on_init` 中通过 `query_service` 找到它。
 
 ```c
-return host->register_service("ai.openai", 1, &g_service);
+return host->register_service("ai_openai", 1, &g_service);
 ```
 
 注册表内的 vtable 是原始指针，不拷贝。因此 vtable 指向的结构体必须是**静态生命周期**（全局变量或 static 局部变量）。