Mastra Studio adds its own SSO, separate from server auth

Mastra Studio can now use a custom SSO provider such as WorkOS, Okta, Auth0, Clerk, or Google, separate from server auth. Setting MASTRA_PLATFORM_STUDIO_AUTH=disabled replaces platform sign-in with the studio.auth provider. Disabling platform sign-in without configuring studio.auth or server.auth leaves Studio publicly accessible, and the change requires @mastra/core 1.42.0 or later.