fix(secret): carry BIP39 passphrase in Unlock protocol variant

The Unlock variant had a single  field used as the
mnemonic, with no way to convey the BIP39 password extension (25th word).
The actor handler silently passed  for the passphrase, making it
impossible to unlock with a BIP39 passphrase via irpc.

Split into  +  to match
the spec and SecretServiceHandle::unlock() signature.

tasks/integration/phase3/secret-service/unlock-passphrase-gap.md

@@ -1,7 +1,7 @@
 ---
 id: unlock-passphrase-gap
 name: Fix Unlock protocol variant to carry both mnemonic and BIP39 passphrase
-status: pending
+status: complete
 depends_on: [irpc-secret-protocol-integration]
 scope: narrow
 risk: low