[fix](iceberg) Fix NPE in COUNT(*) pushdown when snapshot summary omits total-* counters

[raghav-reglobe]

Proposed changes

IcebergScanNode.getCountFromSnapshot() reads total-equality-deletes,
total-position-deletes and total-records from the Iceberg snapshot summary
and calls .equals() / Long.parseLong() directly on the Map.get() results.

An Iceberg snapshot summary is not guaranteed to carry these total-*
counters — snapshots written by compaction/replace (and some writers) may omit
them. When a counter is absent, SELECT COUNT(*) throws:

java.lang.NullPointerException: Cannot invoke "String.equals(Object)" because
the return value of "java.util.Map.get(Object)" is null
    at org.apache.doris.datasource.iceberg.source.IcebergScanNode.getCountFromSnapshot(IcebergScanNode.java:1154)
    at org.apache.doris.datasource.iceberg.source.IcebergScanNode.isBatchMode(...)
    at org.apache.doris.datasource.FileQueryScanNode.createScanRangeLocations(...)

SELECT * on the same table works (it scans); only COUNT(*) fails (it takes
the metadata-count shortcut). Reproducible on current master.

Fix

Extract the summary parsing into a pure static
getCountFromSummary(Map<String, String> summary, boolean ignoreDanglingDelete)
that null-checks the counters and falls back to a normal scan (return -1,
the method's existing "cannot push down count" signal) when any required
counter is absent. Behaviour is otherwise unchanged.

A similar unguarded access exists in IcebergUtils
(Long.parseLong(summary.get(TOTAL_RECORDS)) - Long.parseLong(summary.get(TOTAL_POSITION_DELETES))).
Happy to guard it in this PR or a follow-up — let me know your preference.

Release note

Fix a NullPointerException on SELECT COUNT(*) over an Iceberg table whose
latest snapshot summary omits the total-* counters (e.g. snapshots produced
by compaction/replace).

Check List

• New feature / bug fix has unit test
  (IcebergCountPushDownTest: missing-counter, no-delete, equality-delete and
  position-delete cases)
• Behaviour-preserving for complete summaries; only adds a null-safe
  fall-back to scan

[hello-stephen]

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