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/architecture.md

@@ -90,7 +90,7 @@ struct dstalk_ai_service_t {
 每种服务都有一个预定义的 vtable 结构体（定义在 `dstalk_services.h`）。第三方也可以扩展自己的服务 vtable，版本号随注册一起提供，允许消费者做最低版本检查。
 
 服务注册采用 **name + version** 两要素：
-- 全局唯一名称（如 `"ai.openai"`、`"http"`、`"file_io"`）。
+- 全局唯一名称（如 `"ai_openai"`、`"http"`、`"file_io"`）。
 - 版本号（消费者可以要求 `min_version`）。
 
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