fix(sdk-coin-xtz): verify destination and amount in verifyTransaction

[bitgo-ai-agent-dev]

What

• Xtz.verifyTransaction now decodes txPrebuild.txHex and compares the encoded destination address and transfer amount against txParams.recipients[0]
• Added rejection when the transaction cannot be decoded (fail-closed behaviour)
• Added rejection when the prebuild output count is not exactly 1
• No behaviour change when no recipients or no txHex are present (wallet initialisation)

Why

• A compromised prebuild service could redirect an XTZ transfer to an attacker-controlled address while the local signer signed it silently, because verifyTransaction previously only checked that the recipient count was ≤ 1 and returned true unconditionally
• The multisig contract validates signatures over the payload, but if the payload already encodes an attacker-chosen destination, the on-chain check does not protect the user
• This fix ensures the locally-verified intent (destination + amount from txParams.recipients) matches the bytes that will actually be signed and broadcast

Test plan

• should verify a valid transaction where destination and amount match — passes for a matching prebuild
• should reject when prebuild destination does not match requested recipient — catches address redirection
• should reject when prebuild amount does not match requested recipient — catches amount manipulation
• should reject when the prebuild contains an unexpected number of outputs — catches unexpected multi-output builds
• should reject when the prebuild cannot be decoded — fail-closed on corrupt/invalid txHex
• should pass when no recipients are specified — backward-compatible for wallet-init flows
• should pass when no txHex is present in the prebuild — backward-compatible for wallet-init flows
• All 114 existing unit tests pass (yarn run unit-test --scope @bitgo/sdk-coin-xtz)

Ticket: CSHLD-838

[linear-code]

CSHLD-838