Gitea public-only token bypass through Actions runner registration

Gitea did not reject personal access tokens restricted to public resources (`public-only`) on the organization Actions runner registration-token endpoint, and the runner registered with the resulting token was not limited to public repositories. Holding only such a token with `write:organization` that belongs to an owner of a public organization, an attacker could register an organization-level runner and receive jobs of the organization's private repositories that have Actions enabled, including the workflow content, Actions secrets and variables, and a task token with access to the private repository. The issue widens what the restricted token can reach; it gives nothing beyond what the owner's full account can already do.