Make the GitHub personal-access-token transport fail over across its token pool when one token is rate…

Make the GitHub personal-access-token transport fail over across its token pool when one token is rate…: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). Today MakeGitHubTransport simply assigns one token to each request, so an exhausted token is handed back to the generic…