Wallet: Reject invalid BIP39 unlock

[Tabakkui]

Problem

Raven-Qt can abort with a MinGW runtime assertion in wallet/crypter.cpp:201 when an old encrypted wallet contains BIP39 data that cannot be decrypted. Issue #1252 reports this while attempting to send RVN from a legacy wallet.

Approach

• Treat BIP39 decryption failure during unlock as a normal unlock failure.
• Preserve the existing log message and let the caller report an invalid passphrase/corrupt wallet instead of terminating the GUI.
• Add a wallet crypto regression test using malformed encrypted BIP39 data.

Test plan

• git diff --check
• Static regression assertions for the guarded return path and test case
• Full C++ unit/build test not run on the ARM64 Pi; requires the authorized PC build environment.

Fixes #1252