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/src/ai_common.cpp

@@ -0,0 +1,39 @@
+/*
+ * @file ai_common.cpp
+ * @brief Shared utility implementations for AI provider plugins.
+ * AI 提供者插件的共享工具函数实现。
+ * Copyright (c) 2026 dstalk contributors. GPLv3.
+ */
+
+#include "ai_common.hpp"
+
+namespace dstalk_ai {
+
+void secure_zero(void* p, size_t n) {
+    volatile char* vp = static_cast<volatile char*>(p);
+    while (n--) *vp++ = 0;
+}
+
+bool extract_host_port(const std::string& url,
+    std::string& scheme_out, std::string& host_out,
+    std::string& port_out, std::string& target_out)
+{
+    size_t scheme_end = url.find("://");
+    if (scheme_end == std::string::npos) return false;
+    scheme_out = url.substr(0, scheme_end);
+    std::string rest = url.substr(scheme_end + 3);
+    size_t slash = rest.find('/');
+    std::string authority = (slash != std::string::npos) ? rest.substr(0, slash) : rest;
+    target_out = (slash != std::string::npos) ? rest.substr(slash) : "/";
+    size_t colon = authority.rfind(':');
+    if (colon != std::string::npos) {
+        host_out = authority.substr(0, colon);
+        port_out = authority.substr(colon + 1);
+    } else {
+        host_out = authority;
+        port_out = (scheme_out == "https") ? "443" : "80";
+    }
+    return true;
+}
+
+} // namespace dstalk_ai