Disconnect without resetting YubiKeys if it is safe to do so

This enables the PIN caches to be preserved across age-plugin-yubikey
processes, allowing PIN policies of "once" to become meaningful.

src/key.rs

@@ -236,6 +236,23 @@ pub(crate) fn open(serial: Option<Serial>) -> Result<YubiKey, Error> {
     Ok(yubikey)
 }
 
+/// Disconnect from the YubiKey without resetting it.
+///
+/// This can be used to preserve the YubiKey's PIN and touch caches. There are two cases
+/// where we want to do this:
+///
+/// - We connected to this YubiKey in a read-only context, so we have not made any changes
+///   to the YubiKey's state. However, we might have asked an agent to release the YubiKey
+///   in `key::open_connection`, and we want to allow any state it may have left behind
+///   (such as cached PINs or touches) to persist beyond our execution, for usability.
+/// - We opened this connection in a decryption context, so the only changes to the
+///   YubiKey's state were to potentially cache the PIN and/or touch (depending on the
+///   policies of the slot). We want to allow these to persist beyond our execution, for
+///   usability.
+pub(crate) fn disconnect_without_reset(yubikey: YubiKey) {
+    let _ = yubikey.disconnect(pcsc::Disposition::LeaveCard);
+}
+
 pub(crate) fn manage(yubikey: &mut YubiKey) -> Result<(), Error> {
     const DEFAULT_PIN: &str = "123456";
     const DEFAULT_PUK: &str = "12345678";
@@ -674,6 +691,13 @@ impl Connection {
             Err(_) => Err(()),
         }
     }
+
+    /// Close this connection without resetting the YubiKey.
+    ///
+    /// This can be used to preserve the YubiKey's PIN and touch caches.
+    pub(crate) fn disconnect_without_reset(self) {
+        disconnect_without_reset(self.yubikey);
+    }
 }
 
 #[cfg(test)]