fix(kms): stop an oversized Vault credential window from panicking the request path

refresh_safety_window_secs is operator-supplied and unbounded, so a window like
u64::MAX passed validation and then reached `Instant::now() + safety_window` in
VaultCredentialProvider::current. After a lease-bearing login the first request
panicked with "overflow when adding duration to instant" — reachable from the
admin configure API for every login-based auth method.

Use checked arithmetic and collapse an unrepresentable window to "refuse": such
a window means every token is always inside it, so that is both the fail-closed
answer and the one the arithmetic was reaching for. The comparison moves into
one helper because current() and record_credential_gauges() must apply the same
gate, which their doc comments already require.

The other operand had the same defect: expires_at and renew_at add a TTL built
from the lease_duration the Vault server sent, an unvalidated u64 off the wire.
An unrepresentable TTL now collapses to None, which is indistinguishable from
the no-expiry case Vault already produces for zero-lease tokens; the token stays
in use and Vault still validates it on every call. Leaving that side unchecked
would have kept the same panic reachable through the lease instead of the
window.

crates/kms/src/backends/vault_credentials.rs

@@ -690,15 +690,25 @@ pub(crate) struct VaultClientHandle {
 
 impl VaultClientHandle {
     /// Absolute expiry of this generation's token.
+    ///
+    /// `lease.ttl` is built from the `lease_duration` the Vault server sent, so
+    /// a value too large to add to `issued_at` would panic on the bare `+`. A
+    /// TTL that cannot be represented is indistinguishable from no expiry, so it
+    /// collapses to `None` — the same answer already given for the zero-lease
+    /// tokens Vault issues, which keeps the token in use and still fully
+    /// validated by Vault on every call.
     fn expires_at(&self) -> Option<Instant> {
-        self.lease.map(|lease| self.issued_at + lease.ttl)
+        self.lease.and_then(|lease| self.issued_at.checked_add(lease.ttl))
     }
 
     /// When the renewal task should refresh this generation: half the TTL,
     /// leaving the second half as budget for retries before the fail-closed
     /// window is reached.
+    ///
+    /// Unrepresentable TTLs collapse to `None` as in [`Self::expires_at`],
+    /// leaving a token that never expires with nothing to renew.
     fn renew_at(&self) -> Option<Instant> {
-        self.lease.map(|lease| self.issued_at + lease.ttl / 2)
+        self.lease.and_then(|lease| self.issued_at.checked_add(lease.ttl / 2))
     }
 }
 
@@ -768,7 +778,7 @@ impl VaultCredentialProvider {
         let handle = self.current.load_full();
         if let Some(expires_at) = handle.expires_at() {
             let now = Instant::now();
-            if now + self.policy.safety_window >= expires_at {
+            if self.inside_safety_window(now, expires_at) {
                 return Err(KmsError::credentials_unavailable(format!(
                     "Vault token (generation {}) is within {:?} of expiry and has not been refreshed; refusing to use it",
                     handle.generation, self.policy.safety_window
@@ -778,6 +788,18 @@ impl VaultCredentialProvider {
         Ok(handle)
     }
 
+    /// Whether the token expiring at `expires_at` is close enough to refuse.
+    ///
+    /// `safety_window` reaches here from persisted configuration, so it is not
+    /// guaranteed to have passed this version's validation: a window too large
+    /// to add to the current instant would panic on the bare `+`. Such a window
+    /// means every token is always inside it, so saturating to "refuse" is both
+    /// the fail-closed answer and the one the arithmetic was reaching for.
+    fn inside_safety_window(&self, now: Instant, expires_at: Instant) -> bool {
+        now.checked_add(self.policy.safety_window)
+            .is_none_or(|deadline| deadline >= expires_at)
+    }
+
     /// Publish the credential gauges for the generation currently installed.
     ///
     /// The fail-closed gauge re-evaluates the very gate
@@ -789,7 +811,7 @@ impl VaultCredentialProvider {
         let fail_closed = match handle.expires_at() {
             Some(expires_at) => {
                 metrics::gauge!(METRIC_TOKEN_TTL_SECONDS).set(expires_at.saturating_duration_since(now).as_secs_f64());
-                now + self.policy.safety_window >= expires_at
+                self.inside_safety_window(now, expires_at)
             }
             // A generation without an expiry has no remaining TTL to report
             // and can never lapse, so it can never fail closed either.
@@ -1173,6 +1195,47 @@ mod tests {
         assert!(format!("{source:?}").contains("KubernetesLogin"));
     }
 
+    /// `refresh_safety_window_secs` is operator-supplied and reaches the request
+    /// path from persisted configuration, so the fail-closed comparison must
+    /// survive a window too large to add to the current instant. Before the
+    /// checked arithmetic this panicked with "overflow when adding duration to
+    /// instant" on the first request after a lease-bearing login.
+    #[tokio::test]
+    async fn test_current_refuses_rather_than_panics_on_an_unrepresentable_safety_window() {
+        let (provider, _state) = scripted_provider(
+            Duration::from_secs(60),
+            true,
+            test_policy(Duration::from_secs(u64::MAX), Duration::from_secs(5)),
+        )
+        .await;
+
+        let error = provider
+            .current()
+            .expect_err("a window wider than any lease must refuse the token");
+        assert!(
+            matches!(error, KmsError::CredentialsUnavailable { .. }),
+            "expected CredentialsUnavailable, got {error:?}"
+        );
+    }
+
+    /// `lease_duration` is a bare u64 straight off the Vault response and forms
+    /// the other side of the same comparison, so an absurd one must not panic
+    /// either. It is indistinguishable from a non-expiring token, which is how
+    /// the zero-lease case already behaves.
+    #[tokio::test]
+    async fn test_an_unrepresentable_lease_is_treated_as_non_expiring() {
+        let (provider, _state) = scripted_provider(
+            Duration::from_secs(u64::MAX),
+            true,
+            test_policy(Duration::from_secs(30), Duration::from_secs(5)),
+        )
+        .await;
+
+        provider
+            .current()
+            .expect("a token whose expiry cannot be represented must stay usable");
+    }
+
     /// The configured flag has to reach the HTTP client, not just the config
     /// struct: every generation (authenticated and login) builds its own client,
     /// and a Vault with a self-signed certificate fails the handshake unless