Merge dashpay#6673: fix: use correct hash of quorum as cycle hash for regtest

ff61071 fix: use correct hash of quorum as cycle hash to prevent possible misunderstandings (Konstantin Akimov)

Pull request description:

  ## Issue being fixed or feature implemented
  feature_llmq_singlenode.py works unstable, roughly 10% failure rate. It fails with this error:

      2025-05-17T15:03:08.586000Z TestFramework (INFO): InstantSend lock on tx: 202b6698cf0da8adfeff8430daff3d1cfe767d1120751bb7765f6df15d09a3ee is expecting
      2025-05-17T15:04:09.033000Z TestFramework.utils (ERROR): wait_until() failed. Predicate: ''''
              def check_instantlock():
                  self.bump_mocktime(1)
                  try:
                      return node.getrawtransaction(txid, True)["instantlock"]
                  except:
                      return False
      '''
      2025-05-17T15:04:09.033000Z TestFramework (ERROR): Assertion failed

  Deep in logs the failure look like this:
  ```
   node0 2025-05-17T14:40:45.628148Z (mocktime: 2014-12-12T05:56:24Z) [     isman] [llmq/instantsend.cpp:985] [ProcessPendingInstantSendLocks] [instantsend] CInstantSendManager::ProcessPendingInstantSendLocks -- txid=202b6698cf0da8adfeff8430daff3d1cfe767d1120751bb7765f6df15d09a3ee, islock=89b6a8bb50c2a6b07ebd2bbde0727a72ed4fbe4f78247aab509189b50fc7293f: invalid sig in islock, peer=1
  ```

  Further debug shown that somehow sign-hash sometimes is different; while msgHash is same; signature is valid and bytes-per-bytes matched; just validation code does not work as expected.

  ## What was done?
  For non-rotating quorums on RegTest should be used directly quorum hash instead `cycleQuorum`; no quorum choosing procedure is needed.

  ## How Has This Been Tested?
  Run multiple times functional tests.

  ## Breaking Changes
  N/A; affect only RegTests for special case of non-rotating quorum used for InstantSend.

  ## Checklist:
  - [x] I have performed a self-review of my own code
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added or updated relevant unit/integration/functional/e2e tests
  - [ ] I have made corresponding changes to the documentation
  - [x] I have assigned this pull request to a milestone

ACKs for top commit:
  UdjinM6:
    light ACK ff61071

Tree-SHA512: 7be0eb246f13f851de2c6ce6543f0bac8faf64cbc2a8f5fd6763f0aaca602cfb99e6df882d9e9e8a6d65fbdcdd9188e925427578d86729696dc23652856f7e1c

Author: PastaPastaPasta

src/llmq/instantsend.cpp

@@ -911,8 +911,11 @@ std::unordered_set<uint256, StaticSaltedHasher> CInstantSendManager::ProcessPend
         if (blockIndex->nHeight + dkgInterval < m_chainstate.m_chain.Height()) {
             nSignHeight = blockIndex->nHeight + dkgInterval - 1;
         }
+        // For RegTest non-rotating quorum cycleHash has directly quorum hash
+        auto quorum = llmq_params.useRotation ? llmq::SelectQuorumForSigning(llmq_params, m_chainstate.m_chain, qman,
+                                                                             id, nSignHeight, signOffset)
+                                              : qman.GetQuorum(llmq_params.type, islock->cycleHash);
 
-        auto quorum = llmq::SelectQuorumForSigning(llmq_params, m_chainstate.m_chain, qman, id, nSignHeight, signOffset);
         if (!quorum) {
             // should not happen, but if one fails to select, all others will also fail to select
             return {};