refactor: 提取 Claude 客户端常量到独立包

- 新增 internal/pkg/claude 包统一管理 Claude Code 相关常量
  - 统一账号测试逻辑，所有账号类型使用相同的 Claude Code 风格请求
  - 网关服务使用常量包替换硬编码的 beta header 字符串

backend/internal/pkg/claude/constants.go

@@ -0,0 +1,32 @@
+package claude
+
+// Claude Code 客户端相关常量
+
+// Beta header 常量
+const (
+	BetaOAuth                  = "oauth-2025-04-20"
+	BetaClaudeCode             = "claude-code-20250219"
+	BetaInterleavedThinking    = "interleaved-thinking-2025-05-14"
+	BetaFineGrainedToolStreaming = "fine-grained-tool-streaming-2025-05-14"
+)
+
+// DefaultBetaHeader Claude Code 客户端默认的 anthropic-beta header
+const DefaultBetaHeader = BetaClaudeCode + "," + BetaOAuth + "," + BetaInterleavedThinking + "," + BetaFineGrainedToolStreaming
+
+// HaikuBetaHeader Haiku 模型使用的 anthropic-beta header（不需要 claude-code beta）
+const HaikuBetaHeader = BetaOAuth + "," + BetaInterleavedThinking
+
+// Claude Code 客户端默认请求头
+var DefaultHeaders = map[string]string{
+	"User-Agent":                                "claude-cli/2.0.62 (external, cli)",
+	"X-Stainless-Lang":                          "js",
+	"X-Stainless-Package-Version":               "0.52.0",
+	"X-Stainless-OS":                            "Linux",
+	"X-Stainless-Arch":                          "x64",
+	"X-Stainless-Runtime":                       "node",
+	"X-Stainless-Runtime-Version":               "v22.14.0",
+	"X-Stainless-Retry-Count":                   "0",
+	"X-Stainless-Timeout":                       "60",
+	"X-App":                                     "cli",
+	"Anthropic-Dangerous-Direct-Browser-Access": "true",
+}