Truncate quickinfo LS requests

[weswigham]

To cut down on channel traffic and spend less time doing things the user is likely to discard. @mjbvz recommends a 2000 character cap, with a trailing ... or something if there's more text. We can also approximate this cap while actually building the nodes and use that to estimate when we might be able to bail out of node building early (but still built enough to generate the requisite 2000 characters).

Ref #24140

Oh, @mjbvz can we truncate signature help (#23458), too? Should we have a flag to allow truncation?