feat(key-wallet): add build_and_sign_transaction_with_signer

Mirrors `ManagedWalletInfo::build_and_sign_transaction` but delegates
every input signature to an external `Signer`, so hosts backing an
`ExternalSignable` wallet (hardware/remote signer) never see the
underlying private keys. Same input/output and fee-calculation shape
as the soft path; signs via pre-computed legacy P2PKH sighashes and
requires `SignerMethod::Digest`.

Adds 5 unit tests covering: invalid account index, signer without
digest support, happy-path end-to-end (UTXO-funded wallet, recipient
output present, every input signed), insufficient funds, and
network-mismatched output rejection.

Author: ZocoLini