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>

plugins_upper/ai_common/CMakeLists.txt

@@ -0,0 +1,20 @@
+# ============================================================
+# ai_common — 共享 AI 插件工具库（静态库）/ Shared AI plugin utility library (static)
+# ============================================================
+
+find_package(Boost REQUIRED CONFIG)
+
+add_library(ai_common STATIC
+    src/ai_common.cpp
+)
+
+target_include_directories(ai_common PUBLIC include)
+
+target_compile_features(ai_common PUBLIC cxx_std_20)
+
+target_link_libraries(ai_common
+    PUBLIC
+        dstalk
+        dstalk_boost_config
+        boost::boost
+)