Merge bitcoin/bitcoin#29656: chainparams: Change nChainTx type to uint64_t

bf0efb4fc7 scripted-diff: Modernize naming of nChainTx and nTxCount (Fabian Jahr)
72e5d1be1f test: Add basic check for nChainTx type (Fabian Jahr)
dc2938e979 chainparams: Change nChainTx to uint64_t (Fabian Jahr)

Pull request description:

  This picks up the work from #29331 and closes #29258.

  This simply changes the type and addresses the comments from #29331 by changing the type in all relevant places and removing unnecessary casts. This also adds an extremely simple unit test.

  Additionally this modernizes the name of `nChainTx` which helps reviewers check all use of the symbol and can make silent merge conflicts.

ACKs for top commit:
  maflcko:
    only rebase in scripted-diff, re-ACK bf0efb4fc7 🔈
  glozow:
    reACK bf0efb4fc7 via range-diff

Tree-SHA512: ee4020926d0800236fe655d0c7b127215ab36b553b04d5f91494f4b7fac6e1cfe7ee298b07c0983db5a3f4786932acaa54f5fd2ccd45f2fcdcfa13427358dc3b

src/test/fuzz/utxo_snapshot.cpp

@@ -113,9 +113,9 @@ FUZZ_TARGET(utxo_snapshot, .init = initialize_chain)
             if (index->nHeight == chainman.GetSnapshotBaseHeight()) {
                 auto params{chainman.GetParams().AssumeutxoForHeight(index->nHeight)};
                 Assert(params.has_value());
-                Assert(params.value().nChainTx == index->nChainTx);
+                Assert(params.value().m_chain_tx_count == index->m_chain_tx_count);
             } else {
-                Assert(index->nChainTx == 0);
+                Assert(index->m_chain_tx_count == 0);
             }
         }
         Assert(g_chain->size() == coinscache.GetCacheSize());