fix: enforce mnemonic validation

[gantunesr]

The HdKeyring class accepts mnemonics during deserialization without validating that they are valid BIP39 mnemonics. This means invalid mnemonics (words not in the BIP39 wordlist or invalid checksums) could be passed in and would fail later in the key derivation process with unclear error messages.

This PR adds a isValidMnemonic private method that validates mnemonics using validateMnemonic from @metamask/scure-bip39 before processing them.

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Note

Medium Risk
Adds strict BIP39 validation during deserialize/mnemonic initialization, which can cause previously-accepted (but invalid) secret recovery phrases to fail earlier and may impact restoration flows.

Overview
HdKeyring now validates provided mnemonics against the BIP39 english wordlist and checksum (via validateMnemonic) before deriving the seed, throwing Eth-Hd-Keyring: Invalid secret recovery phrase provided on failure.

Tests were expanded to cover invalid word counts, invalid words, invalid checksums, and mnemonic inputs in Buffer/Uint8Array/serialized forms; the changelog notes the new enforcement.

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