Function compute_refresh_delay

crates/openshell-core/src/grpc_client.rs:406–436

Compute the next refresh delay: 80 % of the time remaining until the current token's `exp`, plus up to 10 % jitter, with a small lower bound for already-expired tokens and capped at 12 h. If the token can't be parsed (legacy/non-JWT bearer) or carries the `exp = 0` non-expiring sentinel, default to 6 h.

(slot: &TokenSlot)

/// (legacy/non-JWT bearer) or carries the `exp = 0` non-expiring sentinel,
/// default to 6 h.
fn compute_refresh_delay(slot: &TokenSlot) -> Duration {
    let token = slot
        .read()
        .ok()
        .and_then(|v| v.to_str().ok().map(str::to_string))
        .unwrap_or_default();
    let bearer = token.strip_prefix("Bearer ").unwrap_or(&token);
    let now_ms = i64::try_from(
        SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .map_or(0, |d| d.as_millis()),
    )
    .unwrap_or(i64::MAX);
    let mut delay_ms = match parse_jwt_exp_ms(bearer) {
        Some(0) | None => 21_600_000,
        Some(exp) => {
            let remaining_ms = exp - now_ms;
            if remaining_ms <= 0 {
                1_000
            } else {
                (remaining_ms * 8 / 10).clamp(1_000, 43_200_000)
            }
        }
    };
    // Up to 10 % jitter, derived deterministically from token bytes so
    // unit tests are reproducible without injecting an RNG.
    let jitter_pct = (token.len() % 10) as u64;
    let jitter_ms = (u64::try_from(delay_ms).unwrap_or(0) * jitter_pct) / 100;
    delay_ms = delay_ms.saturating_add(i64::try_from(jitter_ms).unwrap_or(0));
    Duration::from_millis(u64::try_from(delay_ms).unwrap_or(0))
}

/// Decode the `exp` claim from a JWT without verifying its signature.
/// Returns the expiry in milliseconds since the Unix epoch, or `None` if