bench: clean up migrated descriptor wallets via loader teardown

`MigrateLegacyToDescriptor` returns both a spendable descriptor wallet and a watch‑only wallet.
If these remain attached, their files stay open and on Windows this can hang CI when removing the test directory.

By constructing them via `MakeWalletLoader` (which owns the `WalletContext`), both wallets are automatically unloaded when the loader is destroyed at the end.
This ensures no lingering handles or resource leaks when running the benchmark on CI with `-sanity-check`.

Co-authored-by: furszy <[email protected]>

Author: l0rinc

src/bench/wallet_migration.cpp

@@ -3,14 +3,15 @@
 // file COPYING or https://www.opensource.org/licenses/mit-license.php.
 
 #include <bench/bench.h>
-#include <kernel/chain.h>
 #include <interfaces/chain.h>
+#include <interfaces/wallet.h>
+#include <kernel/chain.h>
 #include <node/context.h>
 #include <test/util/mining.h>
 #include <test/util/setup_common.h>
-#include <wallet/test/util.h>
 #include <wallet/context.h>
 #include <wallet/receive.h>
+#include <wallet/test/util.h>
 #include <wallet/wallet.h>
 
 #include <optional>
@@ -19,11 +20,8 @@ namespace wallet{
 
 static void WalletMigration(benchmark::Bench& bench)
 {
-    const auto test_setup = MakeNoLogFileContext<TestingSetup>();
-
-    WalletContext context;
-    context.args = &test_setup->m_args;
-    context.chain = test_setup->m_node.chain.get();
+    const auto test_setup{MakeNoLogFileContext<TestingSetup>()};
+    const auto loader{MakeWalletLoader(*test_setup->m_node.chain, test_setup->m_args)};
 
     // Number of imported watch only addresses
     int NUM_WATCH_ONLY_ADDR = 20;
@@ -63,12 +61,13 @@ static void WalletMigration(benchmark::Bench& bench)
         batch.WriteKey(pubkey, key.GetPrivKey(), CKeyMetadata());
     }
 
-    bench.epochs(/*numEpochs=*/1).run([&context, &wallet] {
-        util::Result<MigrationResult> res = MigrateLegacyToDescriptor(std::move(wallet), /*passphrase=*/"", context, /*was_loaded=*/false);
-        assert(res);
-        assert(res->wallet);
-        assert(res->watchonly_wallet);
-    });
+    bench.epochs(/*numEpochs=*/1)
+         .run([&] {
+             auto res{MigrateLegacyToDescriptor(std::move(wallet), /*passphrase=*/"", *loader->context(), /*was_loaded=*/false)};
+             assert(res);
+             assert(res->wallet);
+             assert(res->watchonly_wallet);
+         });
 }
 
 BENCHMARK(WalletMigration, benchmark::PriorityLevel::LOW);