fix: fetch root artifact from upstream when missing in auto_fetch mode

When auto_fetch=true and the root artifact doesn't exist locally in a
system project (_pypi), now attempts to fetch it from upstream before
starting dependency resolution. Also fixed a bug where fetched_artifacts
was being redeclared, which would lose the root artifact from the list.

backend/app/dependencies.py

@@ -886,6 +886,9 @@ async def resolve_dependencies_with_fetch(
     when a missing dependency is from a system project (e.g., _pypi), it attempts
     to fetch the package from the corresponding upstream registry.
 
+    If the root artifact itself doesn't exist in a system project, it will also
+    be fetched from upstream before resolution begins.
+
     Args:
         db: Database session
         project_name: Project name
@@ -900,43 +903,94 @@ async def resolve_dependencies_with_fetch(
         DependencyResolutionResponse with all resolved artifacts and fetch status
 
     Raises:
-        DependencyNotFoundError: If the root artifact cannot be found
+        DependencyNotFoundError: If the root artifact cannot be found (even after fetch attempt)
         CircularDependencyError: If circular dependencies are detected
         DependencyConflictError: If conflicting versions are required
     """
-    # Resolve the initial artifact (same as sync version)
+    # Track fetched artifacts for response
+    fetched_artifacts: List[ResolvedArtifact] = []
+
+    # Check if project exists
     project = db.query(Project).filter(Project.name == project_name).first()
+
+    # If project doesn't exist and it's a system project pattern, we can't auto-create it
     if not project:
         raise DependencyNotFoundError(project_name, package_name, ref)
 
+    # Check if package exists
     package = db.query(Package).filter(
         Package.project_id == project.id,
         Package.name == package_name,
     ).first()
-    if not package:
-        raise DependencyNotFoundError(project_name, package_name, ref)
+
+    # Try to resolve the root artifact
+    root_artifact_id = None
+    root_version = None
+    root_size = None
 
     # Handle artifact: prefix for direct artifact ID references
     if ref.startswith("artifact:"):
         artifact_id = ref[9:]
         artifact = db.query(Artifact).filter(Artifact.id == artifact_id).first()
-        if not artifact:
-            raise DependencyNotFoundError(project_name, package_name, ref)
+        if artifact:
             root_artifact_id = artifact.id
             root_version = artifact_id[:12]
             root_size = artifact.size
-    else:
+    elif package:
+        # Try to resolve by version/constraint
         resolved = _resolve_dependency_to_artifact(
             db, project_name, package_name, ref
         )
-        if not resolved:
-            raise DependencyNotFoundError(project_name, package_name, ref)
+        if resolved:
             root_artifact_id, root_version, root_size = resolved
 
+    # If root artifact not found and this is a system project, try to fetch it
+    if root_artifact_id is None and project_name in SYSTEM_PROJECT_REGISTRY_MAP:
+        logger.info(
+            f"Root artifact {project_name}/{package_name}@{ref} not found, "
+            "attempting to fetch from upstream"
+        )
+
+        client = registry_clients.get(project_name)
+        if client:
+            try:
+                # Resolve the version constraint from upstream
+                version_info = await client.resolve_constraint(package_name, ref)
+                if version_info:
+                    # Fetch and cache the package
+                    fetch_result = await client.fetch_package(
+                        package_name, version_info, db, storage
+                    )
+                    if fetch_result:
+                        logger.info(
+                            f"Successfully fetched root artifact {package_name}=="
+                            f"{fetch_result.version} (artifact {fetch_result.artifact_id[:12]})"
+                        )
+                        root_artifact_id = fetch_result.artifact_id
+                        root_version = fetch_result.version
+                        root_size = fetch_result.size
+
+                        # Add to fetched list
+                        fetched_artifacts.append(ResolvedArtifact(
+                            artifact_id=fetch_result.artifact_id,
+                            project=project_name,
+                            package=package_name,
+                            version=fetch_result.version,
+                            size=fetch_result.size,
+                            download_url=f"{base_url}/api/v1/project/{project_name}/{package_name}/+/{fetch_result.version}",
+                        ))
+            except Exception as e:
+                logger.warning(f"Failed to fetch root artifact {package_name}: {e}")
+
+    # If still no root artifact, raise error
+    if root_artifact_id is None:
+        raise DependencyNotFoundError(project_name, package_name, ref)
+
     # Track state
     resolved_artifacts: Dict[str, ResolvedArtifact] = {}
     missing_dependencies: List[MissingDependency] = []
-    fetched_artifacts: List[ResolvedArtifact] = []  # Newly fetched
+    # Note: fetched_artifacts was already initialized above (line 911)
+    # and may already contain the root artifact if it was fetched from upstream
     version_requirements: Dict[str, List[Dict[str, Any]]] = {}
     visiting: Set[str] = set()
     visited: Set[str] = set()