[MM-56031] Add DeleteSidebarCategory method and channel category API tests (#29161)

* Add DeleteSidebarCategory method and channel category API tests

* Add test to verify channels move to default categories when custom category is deleted

* rename channel category tests and remove duplicate context

* TestNotifications fix

---------

Co-authored-by: Fume <contact@fumedev.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/client4.go

@@ -8306,6 +8306,17 @@ func (c *Client4) UpdateSidebarCategoryForTeamForUser(ctx context.Context, userI
 	return cat, BuildResponse(r), nil
 }
 
+// DeleteSidebarCategoryForTeamForUser deletes a sidebar category for a user in a team.
+func (c *Client4) DeleteSidebarCategoryForTeamForUser(ctx context.Context, userId string, teamId string, categoryId string) (*Response, error) {
+	url := fmt.Sprintf("%s/%s", c.userCategoryRoute(userId, teamId), categoryId)
+	r, err := c.DoAPIDelete(ctx, url)
+	if err != nil {
+		return BuildResponse(r), err
+	}
+	defer closeBody(r)
+	return BuildResponse(r), nil
+}
+
 // CheckIntegrity performs a database integrity check.
 func (c *Client4) CheckIntegrity(ctx context.Context) ([]IntegrityCheckResult, *Response, error) {
 	r, err := c.DoAPIPost(ctx, "/integrity", "")