fix(datafusion): show an unreported partition statistic as NULL, not as -1

[sundapeng]

Problem

The three count columns and total_buckets of the $partitions system table were declared
non-nullable, so a statistic the catalog never had reported to it came out as -1 and read like a
measurement: the partition holds -1 rows, -1 files, -1 buckets. Reporting 0 instead would be worse,
because 0 is a real measurement meaning the partition is empty.

This is the same failure mode as the file-level row count fixed in #624 — a placeholder presented as
an exact statistic — arriving through a second entrance, the catalog. A catalog-managed format table
registers partitions without necessarily measuring them, so an unreported statistic is the normal
case there, not an edge case.

Fix

Partition gains UNKNOWN, UNKNOWN_TOTAL_BUCKETS and is_known(), documenting that on the
observation plane any negative value means never measured while zero is an exact zero — and saying
nothing about the delta plane, where a negative value is a decrement to apply. That mirrors what the
crate already does for file-level counts with DataFileMeta::ROW_COUNT_UNKNOWN and
row_count_known().

The four columns become nullable and unknown renders as NULL.

Tests

• spec::partition::tests::test_unknown_is_negative_and_zero_is_a_measurement pins the contract:
  any negative is unknown, zero and positives are measurements.
• sql_context_tests::test_partitions_system_table_shows_unreported_statistics_as_null asserts real
  statistics still arrive as values before flipping the stub catalog to unknown, so a NULL in the
  second half means unknown rather than the column having stopped being populated at all. It also
  checks the partition is still listed — only its statistics are unknown.

Reverting the record_count field to non-nullable makes that test fail with
Column 'record_count' is declared as non-nullable but contains null values, so the assertion is
load-bearing.

spec::partition 32 passed, sql_context_tests 55 passed, system_tables 15 passed.

[QuakeWang]

LGTM