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/lsp/src/lsp_plugin.cpp

@@ -5,7 +5,7 @@
  * Copyright (c) 2026 dstalk contributors. GPLv3.
  */
 
-// plugin-lsp — LSP (Language Server Protocol) 服务 / LSP (Language Server Protocol) service
+// plugin_lsp — LSP (Language Server Protocol) 服务 / LSP (Language Server Protocol) service
 //
 // 自行管理语言服务器子进程，使用 JSON-RPC 2.0 over stdio 通信 / Self-manages language server subprocess, communicates via JSON-RPC 2.0 over stdio.
 // 无外部服务依赖（不依赖 http/config 等其他插件） / No external service dependencies (does not depend on http/config or other plugins).