Partition values for timestamp_ns columns are not converted to nanoseconds #3652

Apache Iceberg version

None

Please describe the bug 🐞

While looking at the partitioning code I noticed that _to_partition_representation (pyiceberg/partitioning.py) doesn't handle
the v3 nano timestamp types.

For a normal TimestampType column, a datetime partition value gets converted to microseconds since epoch. For
TimestampNanoType / TimestamptzNanoType there is no handler, so the value just passes through unchanged:

from datetime import datetime
from pyiceberg.partitioning import _to_partition_representation
from pyiceberg.types import TimestampType, TimestampNanoType

dt = datetime(2020, 1, 1)
_to_partition_representation(TimestampType(), dt)      # 1577836800000000
_to_partition_representation(TimestampNanoType(), dt)  # datetime.datetime(2020, 1, 1, 0, 0) — not converted

That datetime object then ends up in the DataFile.partition record, and the manifest writer expects an int there
(TimestampNanoWriter calls write_int), so writing to a table with an identity partition on a timestamp_ns column would fail.

Users can't hit this today because writing v3 is still blocked (#1551), but it will surface once that lands. It looks like an
oversight #1632 added nano support to transforms, readers/writers and conversions, but partitioning.py was never updated.

Expected: datetime values for nano timestamp columns should convert to nanoseconds since epoch, the same way the microsecond
types convert to micros.

Willingness to contribute

• I can contribute a fix for this bug independently
• I would be willing to contribute a fix for this bug with guidance from the Iceberg community
• I cannot contribute a fix for this bug at this time