fix(esplora): stop paging when the server does not advance the cursor

fix(esplora): stop paging when the server does not advance the cursor

The per-script-pubkey pagination loop paged forward by asking for the
transactions following the last txid of the previous page, but never
checked that the cursor moved. A server that answers every request with
the same full page of 25 transactions kept the cursor in place, so the
loop re-requested that page forever while appending to an unbounded
`Vec`, and `sync`/`full_scan` never returned.

Track every cursor paged from, and return `InvalidResponse` if a full
page ends on one we have already seen. This catches a cursor that repeats
straight away as well as one that cycles (`A -> B -> A`). Erroring rather
than breaking avoids silently handing the caller a truncated transaction
history, which is how `chain_update` already treats a server it cannot
reconcile with.

Closes #2339