Merge bitcoin#35590: test: wallet: BnB incomplete result on attempt-limit success

6ee05c4 test: wallet: BnB incomplete result on attempt-limit success (Bruno Garcia)

Pull request description:

  BnB can return a valid selection before exhausting the search tree, then hit `TOTAL_TRIES` while continuing to look for a better one. Add a unit test for that path using a known exhaustion fixture plus an exact-match coin, and assert the result is marked incomplete via `GetAlgoCompleted() == false`.

  It kills the following mutant:
  ```diff
  diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
  index 8d69957..e4d07415f3 100644
  --- a/src/wallet/coinselection.cpp
  +++ b/src/wallet/coinselection.cpp
  @@ -212,7 +212,7 @@ util::Result<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool

           if (curr_try >= TOTAL_TRIES) {
               // Solution is not guaranteed to be optimal if `curr_try` hit TOTAL_TRIES
  -            result.SetAlgoCompleted(false);
  +            result.SetAlgoCompleted(true);
               break;
           }

  ```

ACKs for top commit:
  yashbhutwala:
    tACK 6ee05c4
  achow101:
    ACK 6ee05c4
  murchandamus:
    ACK 6ee05c4

Tree-SHA512: 32d0cc6da7586abdd195b87459da0a6ebbdcf8115bef41f5e625864bfa9ce8231a58f8d4b61708279ebeaef372e4f3b5fb18eab28a01dad53f043ba083330b3d

Author: achow101

src/wallet/test/coinselection_tests.cpp

@@ -220,6 +220,26 @@ BOOST_AUTO_TEST_CASE(bnb_test)
     }
 }
 
+BOOST_AUTO_TEST_CASE(bnb_exhaustion_with_solution_test)
+{
+    std::vector<OutputGroup> utxo_pool;
+    utxo_pool.reserve(19);
+
+    CAmount selection_target{800'000};
+    // A hard case with no exact-match solution: BnB must still report that the algorithm did not complete once the
+    // search is pushed into the attempt limit, even though it finds a solution within cost_of_change of the target.
+    for (size_t i = 0; i < 19; ++i) {
+        utxo_pool.push_back(MakeCoin(100'000 + i, /*is_eff_value=*/true, default_cs_params));
+    }
+
+    const auto result{SelectCoinsBnB(utxo_pool, selection_target, /*cost_of_change=*/default_cs_params.m_cost_of_change, MAX_STANDARD_TX_WEIGHT)};
+    BOOST_CHECK_MESSAGE(result, "Falsy result in BnB-Success: Exhaust with early solution");
+    BOOST_CHECK(result->GetSelectedEffectiveValue() > selection_target + 28);
+    BOOST_CHECK_EQUAL(result->GetInputSet().size(), 8U);
+    BOOST_CHECK_EQUAL(result->GetSelectionsEvaluated(), 100'000U);
+    BOOST_CHECK(!result->GetAlgoCompleted());
+}
+
 BOOST_AUTO_TEST_CASE(bnb_feerate_sensitivity_test)
 {
     // Create sets of UTXOs with the same effective amounts at different feerates (but different absolute amounts)