Pin the blobstore fallback and cover cross-instance uploads

Why: an unset object store must keep artifact bytes available to the other process, which the store-level test does not execute.
Evidence: cross_artifact_roundtrip failed with EOF on backend c6fb130 and passed on 46073d30 (SELF, isolated u7drill-k4).
Impact: Dev now builds the fixed backend. Second-instance write traffic stays closed.
Refs: modules/backend tests/integration/multi-instance/multi_instance_test.go
Co-authored-by: Cursor <cursoragent@cursor.com>

tests/integration/multi-instance/multi_instance_test.go

@@ -15,10 +15,12 @@ package multiinstance
 import (
 	"bufio"
 	"bytes"
+	"crypto/sha256"
 	"database/sql"
 	"encoding/json"
 	"fmt"
 	"io"
+	"mime/multipart"
 	"net"
 	"net/http"
 	"net/http/cookiejar"
@@ -123,6 +125,19 @@ func TestTwoInstancesOverSharedPG(t *testing.T) {
 		}
 	})
 
+	t.Run("cross_artifact_roundtrip", func(t *testing.T) {
+		// 未配置对象存储时，A 上传的正文必须落在共享本地目录，B 原样取回。
+		payload := []byte("k4-cross-instance-artifact")
+		id := e.uploadArtifact(t, uc.api, projID, "k4.txt", payload)
+		code, raw := e.raw(t, uc.apiB, "GET", "/api/artifacts/"+id+"/content", nil)
+		if code != http.StatusOK {
+			t.Fatalf("B artifact content: status %d, body %.300s", code, raw)
+		}
+		if sha256.Sum256([]byte(raw)) != sha256.Sum256(payload) {
+			t.Fatalf("B downloaded different bytes")
+		}
+	})
+
 	t.Run("cross_event_sse", func(t *testing.T) {
 		lastSeq := e.lastSeq(t, uc.apiB, convID)
 		events := e.subscribe(t, uc.streamB, convID, lastSeq)
@@ -313,6 +328,9 @@ func (e *env) startInstance(t *testing.T, name, addr string) *instance {
 	)
 	log := &lockedBuffer{}
 	cmd.Stdout, cmd.Stderr = log, log
+	// 演练环境可能带有空的对象存储变量。子进程必须按「未配置」启动，
+	// 否则测不到带类型空 *S3 回落本地目录的路径。
+	cmd.Env = withoutBlobstoreS3(os.Environ())
 	// 独立 cwd：serve 会向 ./data 写 skills/SQLite 等，不能落在测试目录。
 	cmd.Dir = t.TempDir()
 	if err := cmd.Start(); err != nil {
@@ -324,6 +342,52 @@ func (e *env) startInstance(t *testing.T, name, addr string) *instance {
 	return inst
 }
 
+func withoutBlobstoreS3(env []string) []string {
+	out := make([]string, 0, len(env))
+	for _, item := range env {
+		if strings.HasPrefix(item, "BLOBSTORE_S3_") {
+			continue
+		}
+		out = append(out, item)
+	}
+	return out
+}
+
+func (e *env) uploadArtifact(t *testing.T, c *http.Client, projectID, name string, payload []byte) string {
+	t.Helper()
+	var buf bytes.Buffer
+	writer := multipart.NewWriter(&buf)
+	part, err := writer.CreateFormFile("file", name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := part.Write(payload); err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatal(err)
+	}
+	req, err := http.NewRequest(http.MethodPost, e.baseFor(c)+"/api/projects/"+projectID+"/uploads", &buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Header.Set("Content-Type", writer.FormDataContentType())
+	resp, err := c.Do(req)
+	if err != nil {
+		t.Fatalf("upload: %v", err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	var info struct {
+		ArtifactID string `json:"artifactId"`
+	}
+	_ = json.Unmarshal(raw, &info)
+	if resp.StatusCode != http.StatusOK || info.ArtifactID == "" {
+		t.Fatalf("upload status %d id %q body %.300s", resp.StatusCode, info.ArtifactID, raw)
+	}
+	return info.ArtifactID
+}
+
 func (e *env) waitHealthy(t *testing.T, inst *instance) {
 	t.Helper()
 	deadline := time.Now().Add(60 * time.Second)