Merge bitcoin#24855: rpc: Fix setwalletflag disabling of flags

88376c6 test: Test for disabling wallet flags (Andrew Chow)
17ab31a rpc, wallet: setwalletflags warnings are optional (Andrew Chow)

Pull request description:

  Trying to disable a wallet flag with `setwalletflag` results in `Internal bug detected: 'std::any_of(m_results.m_results.begin(), m_results.m_results.end(), [ret](const RPCResult& res) { return res.MatchesType(ret); })'`. This occurs because the `warnings` field was not marked as optional. This PR makes `warnings` optional to avoid this error.

  Also added a test case because apparently we didn't already have one.

ACKs for top commit:
  w0xlt:
    ACK 88376c6

Tree-SHA512: 4f5d3bebf0d022a5ad0f75d70c6562a43c7da6e39e9c3118733327d015c435e2c8d5004fdb039d42407dde5b21231a0f8827623d718abf611a1f06c15af5c806

Author: fanquake

src/wallet/rpc/wallet.cpp

@@ -257,7 +257,7 @@ static RPCHelpMan setwalletflag()
                     {
                         {RPCResult::Type::STR, "flag_name", "The name of the flag that was modified"},
                         {RPCResult::Type::BOOL, "flag_state", "The new state of the flag"},
-                        {RPCResult::Type::STR, "warnings", "Any warnings associated with the change"},
+                        {RPCResult::Type::STR, "warnings", /*optional=*/true, "Any warnings associated with the change"},
                     }
                 },
                 RPCExamples{

test/functional/wallet_avoidreuse.py

@@ -118,6 +118,17 @@ def test_persistence(self):
         assert_raises_rpc_error(-8, "Wallet flag is already set to false", self.nodes[0].setwalletflag, 'avoid_reuse', False)
         assert_raises_rpc_error(-8, "Wallet flag is already set to true", self.nodes[1].setwalletflag, 'avoid_reuse', True)
 
+        # Create a wallet with avoid reuse, and test that disabling it afterwards persists
+        self.nodes[1].createwallet(wallet_name="avoid_reuse_persist", avoid_reuse=True)
+        w = self.nodes[1].get_wallet_rpc("avoid_reuse_persist")
+        assert_equal(w.getwalletinfo()["avoid_reuse"], True)
+        w.setwalletflag("avoid_reuse", False)
+        assert_equal(w.getwalletinfo()["avoid_reuse"], False)
+        w.unloadwallet()
+        self.nodes[1].loadwallet("avoid_reuse_persist")
+        assert_equal(w.getwalletinfo()["avoid_reuse"], False)
+        w.unloadwallet()
+
     def test_immutable(self):
         '''Test immutable wallet flags'''
         self.log.info("Test immutable wallet flags")