feat(deploy): 画布 Agent - 独立 yingce-agent 容器并修复运行中断

生产会话不再因 stdout 管道提前关闭被误判失败。设置 YINGCE_AGENT_URL 后只走独立容器，失败不回退到内嵌进程。

backend/internal/agent/runtime/runtime_test.go

@@ -0,0 +1,83 @@
+package runtime
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// fakeRuntime 用一个假的 agent-runtime.mjs 替换真实运行时，只验证 Go 侧的进程与输出处理。
+func fakeRuntime(t *testing.T, script string) {
+	t.Helper()
+	if _, err := exec.LookPath("node"); err != nil {
+		t.Skip("node is not installed")
+	}
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "agent-runtime.mjs"), []byte(script), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("CANVAS_PI_RUNTIME_DIR", dir)
+}
+
+func noopBridge() Bridge {
+	handler := func(context.Context, map[string]json.RawMessage) (any, error) { return map[string]any{"ok": true}, nil }
+	return Bridge{Model: handler, Tool: handler, Event: handler}
+}
+
+// 回归：Wait 早于读完 stdout 时，大量输出后正常退出会被判成
+// "read |0: file already closed"。
+func TestRunDrainsStdoutBeforeWait(t *testing.T) {
+	fakeRuntime(t, `
+const line = JSON.stringify({ event: "progress", pad: "x".repeat(2048) }) + "\n";
+for (let i = 0; i < 2000; i++) process.stdout.write(line);
+process.stdout.write(JSON.stringify({ event: "settled" }) + "\n");
+`)
+	for i := 0; i < 5; i++ {
+		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+		err := Run(ctx, ProcessRequest{Model: map[string]any{"id": "m"}}, noopBridge())
+		cancel()
+		if err != nil {
+			t.Fatalf("attempt %d: normal exit reported as failure: %v", i, err)
+		}
+	}
+}
+
+// runtime_error 之后仍要读到 EOF，并把该错误作为运行结果返回。
+func TestRunReportsRuntimeErrorAfterDrainingOutput(t *testing.T) {
+	fakeRuntime(t, `
+process.stdout.write(JSON.stringify({ event: "runtime_error", message: "boom" }) + "\n");
+const line = JSON.stringify({ event: "progress", pad: "x".repeat(2048) }) + "\n";
+for (let i = 0; i < 500; i++) process.stdout.write(line);
+process.exitCode = 1;
+`)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	err := Run(ctx, ProcessRequest{Model: map[string]any{"id": "m"}}, noopBridge())
+	if err == nil || err.Error() != "boom" {
+		t.Fatalf("want runtime_error message, got %v", err)
+	}
+}
+
+// 子进程只能拿到白名单环境变量，拿不到数据库连接等密钥。
+func TestRunDoesNotLeakBackendEnvironment(t *testing.T) {
+	t.Setenv("DATABASE_URL", "postgres://secret")
+	fakeRuntime(t, `
+if (process.env.DATABASE_URL) {
+  process.stdout.write(JSON.stringify({ event: "runtime_error", message: "leaked" }) + "\n");
+  process.exitCode = 1;
+}
+`)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	if err := Run(ctx, ProcessRequest{Model: map[string]any{"id": "m"}}, noopBridge()); err != nil {
+		if strings.Contains(err.Error(), "leaked") {
+			t.Fatal("backend environment leaked into the Agent runtime")
+		}
+		t.Fatal(err)
+	}
+}