Fix view store test IDs and improve error handling in app layer

- Use model.NewId() for linked property IDs in testUpdateView to fix
  validation failure (IsValid rejects non-UUID strings)
- Fix import grouping in app/view.go (stdlib imports in one block)
- Return 404 instead of 500 when Update/Delete store calls return
  ErrNotFound (e.g. concurrent deletion TOCTOU race)

server/channels/app/view.go

@@ -5,9 +5,8 @@ package app
 
 import (
 	"encoding/json"
-	"net/http"
-
 	"errors"
+	"net/http"
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
@@ -59,6 +58,10 @@ func (a *App) UpdateView(rctx request.CTX, viewID string, patch *model.ViewPatch
 	view.Patch(patch)
 	updated, err := a.Srv().Store().View().Update(view)
 	if err != nil {
+		var nfErr *store.ErrNotFound
+		if errors.As(err, &nfErr) {
+			return nil, model.NewAppError("UpdateView", "app.view.update.app_error", nil, "", http.StatusNotFound).Wrap(err)
+		}
 		return nil, model.NewAppError("UpdateView", "app.view.update.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
@@ -74,6 +77,10 @@ func (a *App) DeleteView(rctx request.CTX, viewID string) *model.AppError {
 	}
 
 	if err := a.Srv().Store().View().Delete(viewID, model.GetMillis()); err != nil {
+		var nfErr *store.ErrNotFound
+		if errors.As(err, &nfErr) {
+			return model.NewAppError("DeleteView", "app.view.delete.app_error", nil, "", http.StatusNotFound).Wrap(err)
+		}
 		return model.NewAppError("DeleteView", "app.view.delete.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 

server/channels/store/storetest/view_store.go

@@ -198,8 +198,10 @@ func testUpdateView(t *testing.T, ss store.Store) {
 		fetched, err := ss.View().Get(saved.Id)
 		require.NoError(t, err)
 
+		propA := model.NewId()
+		propB := model.NewId()
 		fetched.Props = &model.ViewBoardProps{
-			LinkedProperties: []string{"prop-a", "prop-b"},
+			LinkedProperties: []string{propA, propB},
 			Subviews:         []model.Subview{{Title: "Kanban", Type: model.SubviewTypeKanban}},
 		}
 		_, err = ss.View().Update(fetched)
@@ -207,7 +209,7 @@ func testUpdateView(t *testing.T, ss store.Store) {
 
 		refetched, err := ss.View().Get(saved.Id)
 		require.NoError(t, err)
-		assert.Equal(t, []string{"prop-a", "prop-b"}, refetched.Props.LinkedProperties)
+		assert.Equal(t, []string{propA, propB}, refetched.Props.LinkedProperties)
 	})
 
 	t.Run("returns not found for unknown ID", func(t *testing.T) {