[v0.20.x-branch] Backport #11075: lnwallet: prevent transaction pagination overflow

[github-actions]

Backport of #11075

---

Change Description

ListTransactionDetails accepts indexOffset and maxTransactions as
uint32 values and previously added them before clamping the requested
endpoint to the number of available transactions. The GetTransactions RPC
passes both request fields through without restricting their sum.

A sufficiently large limit can therefore wrap the endpoint modulo 2^32.
For example, an offset of one and a limit of math.MaxUint32 produce an
endpoint of zero. If at least two transactions are available, the resulting
txDetails[1:0] operation raises a runtime slice-bounds panic instead of
returning the remaining transactions.

This PR moves pagination into a focused helper. It checks the offset against
the slice length using uint64, calculates the requested endpoint in uint64,
and converts to int only after the value is proven to be within the slice.

The existing pagination behavior is preserved:

• a zero limit returns every transaction from the offset;
• oversized limits are clamped to the available transactions;
• offsets at or beyond the end return an empty page with zero indices.

The wider offset comparison also avoids an unsafe uint32-to-int
conversion on 32-bit systems.

User Impact

Malformed but authorized GetTransactions pagination values no longer reach a
slice-bounds panic. On versions with RPC panic recovery this avoids an internal
RPC error and recovered-panic log; on versions without that recovery it also
prevents a process-level denial of service.

Steps to Test

go test ./lnwallet/btcwallet -run TestPaginateTransactionDetails -count=1
go test -race ./lnwallet/btcwallet -run TestPaginateTransactionDetails -count=1
go test ./lnwallet/btcwallet -count=1

The regression table covers unlimited, bounded, clamped, overflowing,
end-of-list, and maximum-offset requests. Restoring the old uint32 addition
makes the overflow case panic with a slice bounds error.

This change is extracted from the second commit in Boris Nagaev's
additional range fixes gist.

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-11075-to-v0.20.x-branch
git worktree add --checkout .worktree/backport-11075-to-v0.20.x-branch backport-11075-to-v0.20.x-branch
cd .worktree/backport-11075-to-v0.20.x-branch
git reset --hard HEAD^
git cherry-pick -x a207fac5a90394cd0f1e6c4fcf732fc7c93d75d5
git push --force-with-lease

[ziggie1984]

LGTM