Fix peers abruptly disconnecting from AssumeUTXO nodes during IBD

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.