fix(ext/node): use constant-time comparison for secret key equality (#32994)

## Summary
- `KeyObjectHandle::Secret` `PartialEq` impl used standard `==` for byte
slice comparison, which short-circuits on first mismatch
- Switched to `subtle::ConstantTimeEq` (`ct_eq`) to prevent potential
timing side-channels when comparing secret keys via `key.equals()`
- The `subtle` crate was already a dependency of `deno_node_crypto`

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

ext/node_crypto/keys.rs

@@ -411,7 +411,10 @@ impl PartialEq for KeyObjectHandle {
     match (self, other) {
       (Self::AsymmetricPrivate(a), Self::AsymmetricPrivate(b)) => a == b,
       (Self::AsymmetricPublic(a), Self::AsymmetricPublic(b)) => a == b,
-      (Self::Secret(a), Self::Secret(b)) => a == b,
+      (Self::Secret(a), Self::Secret(b)) => {
+        use subtle::ConstantTimeEq;
+        a.ct_eq(b).into()
+      }
       _ => false,
     }
   }