Fix address UTXO endpoint

[MiguelKorTes]

Summary

• Fix getAddressUTXO() to call the confirmed address UTXO endpoint: /address/utxo/{address}
• Remove describe.only from the address wrapper tests so the full suite is not accidentally focused on one file
• Add a mocked regression test for the generated UTXO URL

Verification

• npm run build
• npm test -- --runTestsByPath src/tests/address-url.test.ts --coverage=false

Note: submitted for the Bitgesell bounty/improvement program referenced in BitgesellOfficial/bitgesell#39 and BitgesellOfficial/bitgesell#81. Payout details can be provided after review/approval.

[MyTH-zyxeon]

Maintainers - this looks like a narrow, reviewer-friendly Bitgesell improvement slice.

What seems immediately valuable here:

• getAddressUTXO() now targets /address/utxo/{address}, which matches the method name much better than the old unconfirmed-transactions path.
• The added mocked URL regression test makes the endpoint contract explicit, instead of relying on a live API call.
• Removing describe.only is important here because otherwise the new regression test could pass while the rest of the address test surface is silently skipped.

Suggested acceptance checks:

1. getAddressUTXO(address) now always hits the confirmed UTXO endpoint and returns the same response shape the SDK docs/types expect.
2. The full address test suite still runs after dropping describe.only, not just the new focused URL test.
3. If the old unconfirmed-transactions behavior is still needed anywhere, it should stay as a separate method rather than overloading getAddressUTXO() with ambiguous semantics.

From a reviewability standpoint, this is a good small PR: one endpoint correction, one guardrail test, one test-harness cleanup.