Merge bitcoin/bitcoin#30807: Fix peers abruptly disconnecting from AssumeUTXO nodes during IBD

992f83bb6f test: add coverage for assumeUTXO honest peers disconnection (furszy)
6d5812e5c8 assumeUTXO: fix peers disconnection during sync (furszy)

Pull request description:

  Because AssumeUTXO nodes prioritize tip synchronization, they relay their local
  address through the network before completing the background chain sync.
  This, combined with the advertising of full-node service (`NODE_NETWORK`), can
  result in an honest peer in IBD connecting to the AssumeUTXO node (while syncing)
  and requesting an historical block the node does not have. This behavior leads to
  an abrupt disconnection due to perceived unresponsiveness from the AssumeUTXO
  node.

  This lack of response occurs because nodes ignore `getdata` requests when they do
  not have the block data available (further discussion can be found in #30385).

  Fix this by refraining from signaling full-node service support while the
  background chain is being synced. During this period, the node will only
  signal `NODE_NETWORK_LIMITED` support. Then, full-node (`NODE_NETWORK`)
  support will be re-enabled once the background chain sync is completed.

  Thanks mzumsande for a post-#30385 convo too.

  Testing notes:
  Just cherry-pick the second commit (bb08c22) on master.
  It will fail there, due to the IBD node requesting historical blocks to the snapshot
  node - which is bad because the snapshot node will ignore the requests and
  stall + disconnect after some time.

ACKs for top commit:
  achow101:
    ACK 992f83bb6f
  naumenkogs:
    ACK 992f83bb6f
  mzumsande:
    ACK 992f83bb6f

Tree-SHA512: fef525d1cf3200c2dd89a346be9c82d77f2e28ddaaea1f490a435e180d1a47a371cadea508349777d740ab56e94be536ad8f7d61cc81f6550c58b609b3779ed3

src/rpc/blockchain.cpp

@@ -3042,6 +3042,13 @@ static RPCHelpMan loadtxoutset()
         throw JSONRPCError(RPC_INTERNAL_ERROR, strprintf("Unable to load UTXO snapshot: %s. (%s)", util::ErrorString(activation_result).original, path.utf8string()));
     }
 
+    // Because we can't provide historical blocks during tip or background sync.
+    // Update local services to reflect we are a limited peer until we are fully sync.
+    node.connman->RemoveLocalServices(NODE_NETWORK);
+    // Setting the limited state is usually redundant because the node can always
+    // provide the last 288 blocks, but it doesn't hurt to set it.
+    node.connman->AddLocalServices(NODE_NETWORK_LIMITED);
+
     CBlockIndex& snapshot_index{*CHECK_NONFATAL(*activation_result)};
 
     UniValue result(UniValue::VOBJ);