Select coins until the change output is guaranteed (B-98, BET-24)

select_utxos took the first coin covering the amount, so 10 + 1 PLM and a
10 PLM bet was refused with balance_leaves_no_change and a 10 PLM withdrawal
was reduced. User txs now ask for target + DUST_LIMIT_SATS; the old outcome is
kept only when no allowed combination leaves change.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/unit/test_psbt_builder.py

@@ -279,3 +279,55 @@ def test_dust_sized_recipient_amount_is_refused():
             change_address=change_address,
             fee_rate_sat_vb=1,
         )
+
+
+# --- B-98 (BET-24): selection leaves a change output whenever the coins allow it ---
+
+
+def _built(utxos, amount, reduce=False):
+    signer = _key(7)
+    from_script = script.p2wpkh(signer.to_public())
+    return build_signed_transaction(
+        signing_key=signer,
+        from_script=from_script,
+        utxos=utxos,
+        to_address=script.p2wpkh(_key(8).to_public()).address(network=PLM_MAINNET),
+        amount_sats=amount,
+        change_address=from_script.address(network=PLM_MAINNET),
+        fee_rate_sat_vb=1,
+        reduce_amount_to_keep_change=reduce,
+    )
+
+
+def test_a_bet_takes_a_second_coin_when_the_first_covers_it_exactly():
+    """BET-24: 10 + 1 PLM and a 10 PLM bet — both coins, 1 PLM change, no refusal."""
+    utxos = [Utxo("aa" * 32, 0, 1_000_000_000), Utxo("bb" * 32, 0, 100_000_000)]
+    built = _built(utxos, 1_000_000_000)
+    assert built.change_sats == 100_000_000
+    assert len(Transaction.parse(bytes.fromhex(built.raw_hex)).vin) == 2
+
+
+def test_a_withdrawal_uses_the_second_coin_instead_of_being_reduced():
+    """BET-24 / WDR-09: the reduction only happens when no combination leaves change."""
+    utxos = [Utxo("aa" * 32, 0, 1_000_000_000), Utxo("bb" * 32, 0, 100_000_000)]
+    built = _built(utxos, 1_000_000_000, reduce=True)
+    assert built.change_sats == 100_000_000
+    assert built.recipient_sats == 1_000_000_000 - built.fee_sats
+
+
+def test_with_no_coin_to_spare_the_old_outcome_stands():
+    """BET-24: a lone coin equal to the bet is still refused / the withdrawal still reduced."""
+    from app.wallet.psbt_builder import DUST_LIMIT_SATS
+
+    utxos = [Utxo("aa" * 32, 0, 1_000_000_000)]
+    with pytest.raises(InsufficientFundsError) as excinfo:
+        _built(utxos, 1_000_000_000)
+    assert excinfo.value.code == "balance_leaves_no_change"
+    assert _built(utxos, 1_000_000_000, reduce=True).change_sats == DUST_LIMIT_SATS
+
+
+def test_the_change_target_never_pushes_past_the_input_cap():
+    """BET-24 / BET-08: capped at MAX_TX_INPUTS, a total that covers the amount is kept."""
+    utxos = [Utxo(f"{i:064x}", 0, 100_000) for i in range(200)]
+    selected, total = select_utxos(utxos, 100_000 * MAX_TX_INPUTS, min_change_sats=294)
+    assert len(selected) == MAX_TX_INPUTS and total == 100_000 * MAX_TX_INPUTS