fix(coderd): allow template deletion when only prebuild workspaces remain (#23417)

## Problem

Template administrators cannot delete templates that have running
prebuilds.
The `deleteTemplate` handler fetches all non-deleted workspaces and
blocks
deletion if any exist, making no distinction between human-owned
workspaces
and prebuild workspaces (owned by the system `PrebuildsSystemUserID`).

This forces admins into a manual multi-step workflow: set
`desired_instances`
to 0 on every preset, wait for the reconciler to drain prebuilds, then
retry
deletion. Prebuilds are an internal system concern that admins should
not need
to manage manually.

## Fix

Replace the blanket `len(workspaces) > 0` guard in `deleteTemplate` with
a
loop that only blocks deletion when a non-prebuild (human-owned)
workspace
exists. Prebuild workspaces — owned by `database.PrebuildsSystemUserID`
— are
now ignored during the check.

Once the template is soft-deleted (`deleted=true`), the existing
prebuilds
reconciler detects `isActive()=false` and cleans up remaining prebuilds
asynchronously. No changes to the reconciler are needed.

The error message and HTTP status for human workspaces remain unchanged.

## Testing

Added two new subtests to `TestDeleteTemplate`:
- **`OnlyPrebuilds`**: deletion succeeds when only prebuild workspaces
exist.
- **`PrebuildsAndHumanWorkspaces`**: deletion is blocked when both
prebuild
  and human workspaces exist.

Existing reconciler test ("soft-deleted templates MAY have prebuilds")
already
covers post-deletion prebuild cleanup.

coderd/templates.go

@@ -90,11 +90,17 @@ func (api *API) deleteTemplate(rw http.ResponseWriter, r *http.Request) {
 		})
 		return
 	}
-	if len(workspaces) > 0 {
-		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
-			Message: "All workspaces must be deleted before a template can be removed.",
-		})
-		return
+	// Allow deletion when only prebuild workspaces remain. Prebuilds
+	// are owned by the system user and will be cleaned up
+	// asynchronously by the prebuilds reconciler once the template's
+	// deleted flag is set.
+	for _, ws := range workspaces {
+		if ws.OwnerID != database.PrebuildsSystemUserID {
+			httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+				Message: "All workspaces must be deleted before a template can be removed.",
+			})
+			return
+		}
 	}
 	err = api.Database.UpdateTemplateDeletedByID(ctx, database.UpdateTemplateDeletedByIDParams{
 		ID:        template.ID,

coderd/templates_test.go

@@ -1802,6 +1802,67 @@ func TestDeleteTemplate(t *testing.T) {
 		require.Equal(t, http.StatusForbidden, apiErr.StatusCode())
 	})
 
+	t.Run("OnlyPrebuilds", func(t *testing.T) {
+		t.Parallel()
+		client, db := coderdtest.NewWithDatabase(t, nil)
+		owner := coderdtest.CreateFirstUser(t, client)
+		tpl := dbfake.TemplateVersion(t, db).
+			Seed(database.TemplateVersion{
+				CreatedBy:      owner.UserID,
+				OrganizationID: owner.OrganizationID,
+			}).Do()
+
+		// Create a workspace owned by the prebuilds system user.
+		dbfake.WorkspaceBuild(t, db, database.WorkspaceTable{
+			OwnerID:        database.PrebuildsSystemUserID,
+			OrganizationID: owner.OrganizationID,
+			TemplateID:     tpl.Template.ID,
+		}).Seed(database.WorkspaceBuild{
+			TemplateVersionID: tpl.TemplateVersion.ID,
+		}).Do()
+
+		ctx := testutil.Context(t, testutil.WaitLong)
+
+		err := client.DeleteTemplate(ctx, tpl.Template.ID)
+		require.NoError(t, err)
+	})
+
+	t.Run("PrebuildsAndHumanWorkspaces", func(t *testing.T) {
+		t.Parallel()
+		client, db := coderdtest.NewWithDatabase(t, nil)
+		owner := coderdtest.CreateFirstUser(t, client)
+		tpl := dbfake.TemplateVersion(t, db).
+			Seed(database.TemplateVersion{
+				CreatedBy:      owner.UserID,
+				OrganizationID: owner.OrganizationID,
+			}).Do()
+
+		// Create a prebuild workspace.
+		dbfake.WorkspaceBuild(t, db, database.WorkspaceTable{
+			OwnerID:        database.PrebuildsSystemUserID,
+			OrganizationID: owner.OrganizationID,
+			TemplateID:     tpl.Template.ID,
+		}).Seed(database.WorkspaceBuild{
+			TemplateVersionID: tpl.TemplateVersion.ID,
+		}).Do()
+
+		// Create a human-owned workspace.
+		dbfake.WorkspaceBuild(t, db, database.WorkspaceTable{
+			OwnerID:        owner.UserID,
+			OrganizationID: owner.OrganizationID,
+			TemplateID:     tpl.Template.ID,
+		}).Seed(database.WorkspaceBuild{
+			TemplateVersionID: tpl.TemplateVersion.ID,
+		}).Do()
+
+		ctx := testutil.Context(t, testutil.WaitLong)
+
+		err := client.DeleteTemplate(ctx, tpl.Template.ID)
+		var apiErr *codersdk.Error
+		require.ErrorAs(t, err, &apiErr)
+		require.Equal(t, http.StatusBadRequest, apiErr.StatusCode())
+	})
+
 	t.Run("DeletedIsSet", func(t *testing.T) {
 		t.Parallel()
 		client := coderdtest.New(t, &coderdtest.Options{IncludeProvisionerDaemon: true})