fix(lint): 修复 golangci-lint errcheck 与 gofmt 报错

- gateway_service.go buildStableSessionSeed: strings.Builder.WriteString
  返回值用 _, _ = 显式忽略（errcheck）
- openai_codex_transform_additions_test.go: client_metadata 类型断言改为
  双值形式 cm3, _ := ...（errcheck check-type-assertions）
- openai_gateway_service.go: codexCLIUserAgent 常量单独成组，收敛 = 前多余
  空格对齐（gofmt）

backend/internal/service/gateway_service.go

@@ -1436,11 +1436,11 @@ func (s *GatewayService) buildOAuthMetadataUserIDFromBody(
 // accountID 恒存在，故 seed 永不为空 —— 输出始终是确定性 UUID，而非随机值。
 func buildStableSessionSeed(accountID int64, clientDiscriminator, firstUserText string) string {
 	var b strings.Builder
-	b.WriteString(strconv.FormatInt(accountID, 10))
-	b.WriteString("::")
-	b.WriteString(clientDiscriminator)
-	b.WriteString("::")
-	b.WriteString(firstUserText)
+	_, _ = b.WriteString(strconv.FormatInt(accountID, 10))
+	_, _ = b.WriteString("::")
+	_, _ = b.WriteString(clientDiscriminator)
+	_, _ = b.WriteString("::")
+	_, _ = b.WriteString(firstUserText)
 	return b.String()
 }
 

backend/internal/service/openai_codex_transform_additions_test.go

@@ -53,7 +53,7 @@ func TestApplyCodexClientMetadata(t *testing.T) {
 	// 既有 client_metadata（如 turn metadata）保留，仅补 installation 键
 	body3 := map[string]any{"client_metadata": map[string]any{"x-codex-turn-metadata": "t"}}
 	require.True(t, applyCodexClientMetadata(body3, acc))
-	cm3 := body3["client_metadata"].(map[string]any)
+	cm3, _ := body3["client_metadata"].(map[string]any)
 	require.Equal(t, "t", cm3["x-codex-turn-metadata"])
 	require.Equal(t, "dev-xyz", cm3["x-codex-installation-id"])
 }

backend/internal/service/openai_gateway_service.go

@@ -45,7 +45,7 @@ const (
 	// 与真实 Codex CLI 的 User-Agent 结构对齐：
 	// {originator}/{version} ({OS} {OS_version}; {arch}) {terminal}
 	// 旧值 "codex_cli_rs/0.125.0" 缺少 OS/架构/终端后缀，易被上游指纹识别为非官方客户端。
-	codexCLIUserAgent      = "codex_cli_rs/0.125.0 (Ubuntu 22.4.0; x86_64) xterm-256color"
+	codexCLIUserAgent = "codex_cli_rs/0.125.0 (Ubuntu 22.4.0; x86_64) xterm-256color"
 	// codex_cli_only 拒绝时单个请求头日志长度上限（字符）
 	codexCLIOnlyHeaderValueMaxBytes = 256