An opaque, redacting cached installation access token and its expiry.

This is the value GhEx mints on a cache miss and the value a GhEx.TokenCache implementation stores and returns. The :token is a secret, so it is excluded from Inspect: it never appears in inspect/1, IEx echoes, or an ETS-cache crash dump. The struct still matches %{token: _, expires_at: _}, so a cache implementation and the freshness check can treat it as a plain map.

A custom cache never constructs a Value; it receives one from the GhEx-supplied mint and handles it opaquely. Keeping the public GhEx.TokenCache.value/0 type tied to this struct (rather than a bare map) preserves the redacting shape on the mint path.