Merge bitcoin/bitcoin#33119: rpc: Fix 'getdescriptoractivity' RPCHelpMan, add test to verify fix

3543bfdfec test: Fix 'getdescriptoractivity' RPCHelpMan, add test to verify 'spend_vin' is the correct field (Chris Stewart)

Pull request description:

  Fixes bug in `getdescriptoractivity` RPC help manual.

  Here is the line that pushes `spend_vin` field, there is no `spend_vout` json field.

  https://github.com/bitcoin/bitcoin/blob/master/src/rpc/blockchain.cpp#L2757

ACKs for top commit:
  nervana21:
    tACK 3543bfd
  luke-jr:
    utACK 3543bfdfec
  jonatack:
    ACK 3543bfdfec

Tree-SHA512: 2cd543569a87261d8d804d9afe36f8e8ead55839c01da9c4831aea3ced7d1251e6885621e628898105700aae4d76cbb8a682f518f33c1c52163e66f75ec87a61

test/functional/rpc_getdescriptoractivity.py

@@ -191,6 +191,7 @@ class GetBlocksActivityTest(BitcoinTestFramework):
 
         assert result['activity'][2]['type'] == 'spend'
         assert result['activity'][2]['spend_txid'] == sent2['txid']
+        assert result['activity'][2]['spend_vin'] == 0
         assert result['activity'][2]['prevout_txid'] == sent1['txid']
         assert result['activity'][2]['blockhash'] == blockhash_2