getLedgers: Add bounds check for empty ledger result

[karthikiyer56]

Missing bounds check in getLedgers can cause a panic when the in-memory cache is ahead of the read transaction's visible data. The panic is recovered by the request duration limiter - server continues running, client receives a -32003 error.

Bug

In cmd/stellar-rpc/internal/methods/get_ledgers.go:101:

cursor := strconv.Itoa(int(ledgers[len(ledgers)-1].Sequence))  // panics if len(ledgers) == 0

This can happen during a race where NewTx() captures a cached latestLedgerSeq that the read transaction's snapshot doesn't yet contain.

Fix

if len(ledgers) == 0 {
    return protocol.GetLedgersResponse{}, &jrpc2.Error{
        Code:    jrpc2.InternalError,
        Message: "no ledgers found in requested range",
    }
}

Things to note

• Not a security vulnerability - the panic is recovered, request fails gracefully
• Operators running public endpoints should ensure rate limiting and log rotation are configured appropriately