Issue 2422: Fix SyncLedgerIterator.hasNext() failing to iterate across ZK ledger ranges

[geniusjoe]

Fix #2422

Related PR: #2457

Motivation

The SyncLedgerIterator.hasNext() method in BookKeeper.java has a logic bug that causes it to stop iterating prematurely when ledgers span across multiple ZooKeeper ranges.

ZooKeeper stores ledgers in a hierarchical tree structure (e.g. /ledgers/00/0000/...). Each range (znode) holds at most 10,000 leaf-level ledger IDs.
The 10,000 limit is determined by the 4-digit last level (0000-9999) as defined in LegacyHierarchicalLedgerManager (2-4-4 split) and LongHierarchicalLedgerManager (3-4-4-4-4 split). When the number of ledgers exceeds 10,000, they are distributed across multiple ranges.

The original hasNext() implementation had the following logic:

@Override
public boolean hasNext() throws IOException {
    parent.checkClosed();
    if (currentRange != null) {
        if (currentRange.hasNext()) {
            return true;
        }
    } else if (iterator.hasNext()) {
        return true;
    }
    return false;
}

When currentRange is non-null but exhausted (no more elements), it falls through to return false without checking iterator.hasNext() for more ranges.

Changes

1. Fix SyncLedgerIterator.hasNext() in BookKeeper.java:
   Simplified the conditional logic to properly fall through to iterator.hasNext() when the current range is exhausted
2. Add testListLedgersLargeScale() test in LedgerMetadataTest.java
   Added a new test case with numOfLedgers = 10010 (> 10,000 per-range limit) to verify cross-range iteration works correctly.

[hanmz]

lgtm

[lhotari]

LGTM, good catch

[wenbingshen]

LGTM