Function mint_session_token 

pub async fn mint_session_token<H, C>(
    http: &H,
    clock: &C,
    github_token: &str,
) -> Result<SessionToken, CopilotAuthError>
where
    H: CopilotHttp,
    C: CopilotClock,

Exchange a long-lived GitHub user token for a short-lived Copilot session.

The GitHub token is sent only to the pinned mint URL. Callers must never log the token argument.