fix(iac-workflow): change TF_STATE_REGION fallback from us-east-1 to auto

Cloudflare R2 S3-compatible backend requires region=auto; the previous
fallback us-east-1 would cause terraform init to fail if Vault key is absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/deploy-ai-workspace-iac.yaml

@@ -203,7 +203,7 @@ jobs:
             terraform init -input=false \
               -backend-config="bucket=${TF_STATE_BUCKET}" \
               -backend-config="key=ai-workspace/terraform.tfstate" \
-              -backend-config="region=${TF_STATE_REGION:-us-east-1}"
+              -backend-config="region=${TF_STATE_REGION:-auto}"
           else
             echo "::warning::未配置远端 state(Vault 无 TF_STATE_BUCKET)，使用本地 state（仅适合一次性演示，destroy 需同一次运行）"
             terraform init -input=false