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_base/config/CMakeLists.txt

@@ -1,8 +1,8 @@
-add_library(plugin-config SHARED src/config_plugin.cpp)
+add_library(plugin_config SHARED src/config_plugin.cpp)
 
-target_link_libraries(plugin-config PRIVATE dstalk)
+target_link_libraries(plugin_config PRIVATE dstalk)
 
-set_target_properties(plugin-config PROPERTIES
+set_target_properties(plugin_config PROPERTIES
     PREFIX ""
     LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"
     RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"