test(dash-spv): cover spending mempool change and unconfirmed incoming UTXOs

Adds two regression tests in `tests/dashd_sync/tests_transaction_builder.rs`
that drive `ManagedWalletInfo::build_and_sign_transaction` end-to-end against
a real dashd regtest node, broadcasting via the SPV client.

Both tests use a fresh-mnemonic wallet (EMPTY/SECONDARY) so the SPV side has
no preexisting confirmed UTXOs from the test chain that would mask the bug.

- test_spend_unconfirmed_change_balance: confirms the wallet reports trusted
  mempool change as spendable, then fails to actually spend it through coin
  selection (the filter ignores `is_trusted`).
- test_spend_unconfirmed_incoming_balance: confirms the wallet reports
  unconfirmed incoming as spendable, then fails to spend it.

Both tests are red against the current coin selector and document the user-
reported symptom ("balance shows up but cannot be spent").

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ZocoLini

dash-spv/tests/dashd_sync/main.rs

@@ -10,3 +10,4 @@ mod tests_mempool;
 mod tests_multi_wallet;
 mod tests_restart;
 mod tests_transaction;
+mod tests_transaction_builder;