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/reference/plugin-abi.md

@@ -110,7 +110,7 @@ Linux/macOS 通常共享 libc，但静态链接或不同 libc 版本时同样可
 ### 4.2 重复注册
 
 同一 `name` 不可重复注册：第二次调用返回 `-2`（`service_registry.cpp:13`）。插件应检查返回
-值，在共享服务名（如 `"ai.openai"`）的场景中避免冲突。
+值，在共享服务名（如 `"ai_openai"`）的场景中避免冲突。
 
 ### 4.3 版本协商