[python] Render min/max_partition_stats in the $manifests system table

[wombatu-kun]

Purpose

This closes a TODO in pypaimon/table/system/manifests_table.py:

# TODO: render min/max_partition_stats by casting partition
# rows to their string form. pypaimon
# has SimpleStats but no shared partition-row-to-string
# helper yet; emit NULL to preserve the column shape.
min_partition_stats.append(None)
max_partition_stats.append(None)

So $manifests returns NULL for min_partition_stats and max_partition_stats in pypaimon while Java renders them, and inspecting which manifests cover which partitions gives nothing on the Python side. The data is already parsed: ManifestListManager builds SimpleStats from the manifest list, only the rendering was missing.

This adds cast_row_to_string, the helper the TODO asks for: a port of RowToStringCastRule and the per type *ToStringCastRule rules of paimon-common, with fields joined by ", " inside braces, the literal null for a null field, and {} for an unpartitioned table. The types that need more than str() are boolean (lower case), binary (decoded as UTF-8), date, time, timestamp and float. Timestamps follow DateTimeUtils.formatTimestamp, which separates date and time with a space and strips trailing zeros of the fraction down to the declared precision, so datetime.isoformat() can not be used. Floats print the shortest text that round trips as float32, otherwise a float32 0.1 read into a Python float prints as 0.10000000149011612.

One point for a maintainer opinion, deliberately not changed here: pypaimon renders the partition column of $files and $buckets as pt=v/pt2=v2, while Java renders those two with the brace form and keeps pt=v only for $partitions. Aligning them would change values these tables already return, so it is out of scope for this PR.

Tests

New pypaimon/tests/row_to_string_test.py covers the rendering per type, including a null field, an empty row, the timestamp fraction at precision 0/3/6 and the float32 shortest form.

pypaimon/tests/system/manifests_table_test.py replaces the assertions that pinned the NULL placeholder: an unpartitioned table now yields {}, and a table partitioned by INT and STRING yields {1, 2024-01-01} and {2, 2024-01-02}.

Cross checked against Java on a single warehouse: a table written by pypaimon and partitioned by INT, STRING, DATE, BOOLEAN and DECIMAL renders identically on both sides.