examples(multi-tenant): fix ensure unique slug

[vaclav-cerny]

Page slug uniqueness check does not work in multi-tenant example when the slug or tenant does not exist (in case of creating a new page or using drafts), which will break the page save.

[github-actions[bot]]

Pull Request titles must follow the Conventional Commits specification and have valid scopes.

Unknown scope "multi-tenant" found in pull request title "examples(multi-tenant): fix ensure unique slug". Scope must match one of: cpa, db-*, db-mongodb, db-postgres, db-vercel-postgres, db-sqlite, drizzle, email-nodemailer, eslint, graphql, live-preview, live-preview-react, next, payload-cloud, plugin-cloud, plugin-cloud-storage, plugin-form-builder, plugin-nested-docs, plugin-redirects, plugin-search, plugin-sentry, plugin-seo, plugin-stripe, richtext-*, richtext-lexical, richtext-slate, storage-*, storage-azure, storage-gcs, storage-uploadthing, storage-vercel-blob, storage-s3, translations, ui, templates, examples(/(\w|-)+)?, deps.

feat(ui): add Button component
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