branch-4.1: [fix](iceberg) Fix NPE in COUNT(*) pushdown when snapshot summary omits total-* counters (#64648)

[morningman]

Proposed changes

Backport of #64648 to branch-4.1.

IcebergScanNode.getCountFromSnapshot() read total-equality-deletes /
total-position-deletes / total-records from the Iceberg snapshot summary and
called .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, or some writers, may omit them), so
SELECT COUNT(*) threw a NullPointerException on such tables while SELECT *
worked fine.

Fix

• Extract the summary parsing into a pure static
  IcebergUtils.getCountFromSummary(summary, ignoreDanglingDelete) that
  null-checks the counters and falls back to a normal scan (return -1) when any
  required counter is absent, and reuse it from both IcebergScanNode and
  IcebergUtils.getIcebergRowCount().
• BE: IcebergTableReader::init_row_filters() accepts a table-level row count of
  0 (>= 0 instead of > 0) so a genuine pushed-down count of 0 takes the
  CountReader fast path. Applied to this branch's
  be/src/format/table/iceberg_reader.cpp (master's
  iceberg_reader_mixin.h does not exist on this branch).

Unit test IcebergCountPushDownTest covers the missing-counter, no-delete,
equality-delete, position-delete and zero-count cases.

[hello-stephen]

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[morningman]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 68.42% (13/19) 🎉
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