Merge bitcoin/bitcoin#26194: rpc, wallet: use the same next_index key in listdescriptors and importdescriptors

b082f28 rpc, wallet: use the same `next_index` in listdescriptors and importdescriptors (w0xlt)

Pull request description:

  Currently `listdescriptors` RPC uses `next` key to represent `WalletDescriptor::next_index` while `importdescriptors` uses `next_index`. This creates two different descriptor formats.

  This  PR changes `listdescriptors` to use the same key as `importdescriptors`.

ACKs for top commit:
  achow101:
    ACK b082f28
  aureleoules:
    reACK b082f28

Tree-SHA512: c29ec59051878e614d749ed6dc85e5c14ad00db0e8fcbce3f5066d1aae85ef07ca70f02920299e48d191b7387024fe224b0054c4191a5951cb805106f7b8e37b

Author: achow101

doc/release-note-26194.md

@@ -0,0 +1,4 @@
+Add `next_index` in `listdescriptors` RPC
+-----------------
+
+- Added a new `next_index` field in the response in `listdescriptors` to have the same format as `importdescriptors`

src/wallet/rpc/backup.cpp

@@ -1760,7 +1760,8 @@ RPCHelpMan listdescriptors()
                         {RPCResult::Type::NUM, "", "Range start inclusive"},
                         {RPCResult::Type::NUM, "", "Range end inclusive"},
                     }},
-                    {RPCResult::Type::NUM, "next", /*optional=*/true, "The next index to generate addresses from; defined only for ranged descriptors"},
+                    {RPCResult::Type::NUM, "next", /*optional=*/true, "Same as next_index field. Kept for compatibility reason."},
+                    {RPCResult::Type::NUM, "next_index", /*optional=*/true, "The next index to generate addresses from; defined only for ranged descriptors"},
                 }},
             }}
         }},
@@ -1837,6 +1838,7 @@ RPCHelpMan listdescriptors()
             range.push_back(info.range->second - 1);
             spk.pushKV("range", range);
             spk.pushKV("next", info.next_index);
+            spk.pushKV("next_index", info.next_index);
         }
         descriptors.push_back(spk);
     }

test/functional/wallet_listdescriptors.py

@@ -54,7 +54,7 @@ def run_test(self):
         assert_equal(4, len([d for d in result['descriptors'] if d['internal']]))
         for item in result['descriptors']:
             assert item['desc'] != ''
-            assert item['next'] == 0
+            assert item['next_index'] == 0
             assert item['range'] == [0, 0]
             assert item['timestamp'] is not None
 
@@ -78,7 +78,8 @@ def run_test(self):
                  'timestamp': TIME_GENESIS_BLOCK,
                  'active': False,
                  'range': [0, 0],
-                 'next': 0},
+                 'next': 0,
+                 'next_index': 0},
             ],
         }
         assert_equal(expected, wallet.listdescriptors())
@@ -92,7 +93,8 @@ def run_test(self):
                  'timestamp': TIME_GENESIS_BLOCK,
                  'active': False,
                  'range': [0, 0],
-                 'next': 0},
+                 'next': 0,
+                 'next_index': 0},
             ],
         }
         assert_equal(expected_private, wallet.listdescriptors(True))