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/openai/CMakeLists.txt

@@ -1,19 +1,19 @@
 # ============================================================
-# plugin-openai — OpenAI 兼容 AI 服务 / OpenAI-compatible AI service
+# plugin_openai — OpenAI 兼容 AI 服务 / OpenAI-compatible AI service
 # ============================================================
 
 find_package(Boost REQUIRED CONFIG)
 
-add_library(plugin-openai SHARED
+add_library(plugin_openai SHARED
     src/openai_plugin.cpp
 )
 
-target_link_libraries(plugin-openai PRIVATE dstalk)
+target_link_libraries(plugin_openai PRIVATE dstalk ai_common)
 
 # Boost.JSON (header-only)
-target_link_libraries(plugin-openai PRIVATE boost::boost dstalk_boost_config)
+target_link_libraries(plugin_openai PRIVATE boost::boost dstalk_boost_config)
 
-set_target_properties(plugin-openai PROPERTIES
+set_target_properties(plugin_openai PROPERTIES
     PREFIX ""
     LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/plugins
     RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/plugins