Fix replicas-check counters being reset per ledger range

[SongOf]

Descriptions of the changes in this PR:

Motivation

AuditorReplicasCheckTask.replicasCheck() scans all ledger metadata in LedgerRange
chunks. At the top of every range iteration it reset the three
numLedgersFoundHaving{NoReplica,LessThanAQ,LessThanWQ}OfAnEntry counters to 0, but
runTask() reads them and publishes the gauges only once, after the whole run completes.

As a result, a finding in an earlier range was wiped by any later range, so the
NUM_LEDGERS_HAVING_NO_REPLICA_OF_AN_ENTRY /
NUM_LEDGERS_HAVING_LESS_THAN_AQ_REPLICAS_OF_AN_ENTRY /
NUM_LEDGERS_HAVING_LESS_THAN_WQ_REPLICAS_OF_AN_ENTRY gauges reflected only the last
ledger range and severely undercounted ledgers with missing / under-replicated entries
whenever ledgers span more than one range. Monitoring and alerting built on these gauges
is therefore misleading.

Changes

• Move the three counter resets to once before the range loop so they accumulate across
  all ledger ranges. The ledgersWithMissingEntries / ledgersWithUnavailableBookies maps
  remain per-range scratch state (still cleared each iteration and reported at the end of
  that iteration).
• Add AuditorReplicasCheckTaskCrossRangeTest, a focused Mockito unit test (no cluster,
  fully deterministic): two ledger ranges, an unhealthy ledger below write-quorum in the
  first range and a healthy ledger in the second. It fails on the unpatched code
  (gauge 0 instead of 1) and passes after the fix.

This is an internal metric-accounting fix; it does not change any public API, schema, wire
protocol, REST endpoint, CLI option, or configuration.

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