wallet: check WriteMasterKey result when changing passphrase #35500

CWallet::ChangeWalletPassphrase ignores the return value of WalletBatch::WriteMasterKey. If the database write fails (e.g. disk full, I/O error), the RPC reports success while the wallet file on disk still requires the old passphrase. A user who discards the old passphrase after such a silent failure is locked out of their wallet after a restart.

This PR:

• Re-encrypts into a copy of the CMasterKey and only commits it to mapMasterKeys after the database write succeeds, so in-memory state never diverges from disk in any failure path. (Previously a failing EncryptMasterKey could also leave a mutated nDeriveIterations in the map entry.)
• Checks the WriteMasterKey return value and returns false on failure.
• Logs the write failure and restores the previous lock state before returning, so a failed call leaves the wallet exactly as it was.

The disk-before-memory commit ordering mirrors the intent of EncryptWallet, which already treats memory/disk divergence of key material as unacceptable.

Tested with wallet_tests/wallet_crypto_tests unit tests and the wallet_encryption.py functional test.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35500.

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