PAX: fix SIGSEGV in SUM stats merge during DELETE on minmax columns

[gongxun0928]

Fixes #1767

What does this PR do?

Fixes a segment crash (SIGSEGV) that occurs when PAX tables with minmax_columns containing numeric types (int, bigint, numeric) undergo repeated DELETE operations. The crash happens in datumCopy() during the visibility-map statistics refresh path (MicroPartitionStats::MergeRawInfo()).

Two bugs are fixed in contrib/pax_storage/src/cpp/storage/micro_partition_stats.cc:

1. Wrong type metadata in FromValue(): The serialized SUM value was deserialized using the column's physical type (typlen/typbyval) instead of the SUM aggregate's return type (rettyplen/rettypbyval). For example, sum(bigint) returns numeric, so the stored SUM datum must be interpreted as numeric, not as int8. This mismatch produced an invalid Datum that crashed in datumCopy().

2. Swapped arguments in datumCopy(): The call passed (value, typlen, typbyval) but the wrapper signature is datumCopy(value, typByVal, typLen). This swapped pass-by-value/pass-by-reference flags, leading to memory corruption.

Type of Change

• Bug fix (non-breaking change)

Breaking Changes

None.

Test Plan

• Unit tests added/updated — new regression test delete_sum_stats covering DELETE + INSERT + DELETE on int, bigint, and numeric columns with minmax_columns enabled
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:
None — fix only affects correctness of stats merging, not the hot path.

User-facing changes:
Fixes a crash that users could hit with DELETE on PAX tables with minmax_columns.

Dependencies:
None.

Checklist

• Followed contribution guide
• Added/updated documentation (regression test included)
• Reviewed code for security implications
• Requested review from cloudberry committers

Additional Context

Root cause analysis and reproduction steps are documented in issue #1767. The crash stack trace shows the failure path:

libpostgres.so datumCopy
pax.so cbdb::datumCopy
pax.so pax::MicroPartitionStats::MergeRawInfo
pax.so pax::MicroPartitionStatsUpdater::Update
pax.so pax::TableDeleter::UpdateStatsInAuxTable
pax.so pax::TableDeleter::DeleteWithVisibilityMap