fix: use address UTXO endpoint

[manuelsampedro1]

GetAddressUTXO currently calls the unconfirmed transactions endpoint, so callers requesting UTXOs receive transaction-history data instead of the address UTXO response.

Changes:

• route GetAddressUTXO to /address/utxo/{address}
• update the unit test mock path so the regression is covered

Cross-checks:

• the wallet client in this org already uses /address/utxo/{address} for UTXOs
• bglapiserver exposes the address UTXO route separately from unconfirmed transactions

Validation:

• static endpoint cross-check with rg across the toolkit, wallet client, and bglapiserver route usage

Note: I could not run go test ./... in this local environment because the Go toolchain is not installed here.

Bounty context: BitgesellOfficial/bitgesell#39

[MyTH-zyxeon]

Review-assist note for the Bitgesell #81 improvement lane.

I checked the public PR metadata and diff. This is a focused Go SDK endpoint fix:

• changes GetAddressUTXO from the unconfirmed-transaction endpoint to address/utxo/{address}
• updates the matching unit-test mock route so the test exercises the intended UTXO URL
• keeps the change to Address.go and Address_test.go, with no wallet, signing, RPC credential, or live-chain behavior in this review note

Suggested maintainer checks before merge:

• confirm address/utxo/{address} is the canonical current bglapi route for spendable UTXOs
• add or verify response-shape coverage if AddressUTXOs expects fields that differ from the previous unconfirmed-transaction payload
• run the Go test suite with the mocked endpoint path and, if available, one non-secret dry-run against a local/dev bglapi fixture
• check whether any README or example code still documents the old unconfirmed endpoint for GetAddressUTXO

From the public patch shape, this looks like a useful low-risk SDK correctness candidate for the #81 improvement queue. I did not run Go tests, connect to a node, use wallet keys, submit payment details, or perform any live-chain action for this review-assist note.