fix: 恢复 PR #682 中被误替换为占位符的 OAuth client_secret

PR #682 (release → main 全量同步) 将 Antigravity 和 Gemini CLI 的
OAuth client_secret 硬编码值替换为了 "GOCSPX-your-client-secret" 占位符，
导致未配置环境变量的部署环境中 token 刷新失败。

恢复内容：
- antigravity/oauth.go: 恢复真实 client_secret
- antigravity/oauth_test.go: 恢复测试断言中的真实值
- geminicli/constants.go: 恢复真实 client_secret

backend/internal/pkg/geminicli/constants.go

@@ -39,7 +39,7 @@ const (
 	// They enable the "login without creating your own OAuth client" experience, but Google may
 	// restrict which scopes are allowed for this client.
 	GeminiCLIOAuthClientID     = "681255809395-oo8ft2oprdrnp9e3aqf6av3hmdib135j.apps.googleusercontent.com"
-	GeminiCLIOAuthClientSecret = "GOCSPX-your-client-secret"
+	GeminiCLIOAuthClientSecret = "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"
 
 	// GeminiCLIOAuthClientSecretEnv is the environment variable name for the built-in client secret.
 	GeminiCLIOAuthClientSecretEnv = "GEMINI_CLI_OAUTH_CLIENT_SECRET"