fix: convert nano timestamps to nanoseconds in partition representation

[mittalpk]

Summary

_to_partition_representation() in pyiceberg/partitioning.py has no registered handler for TimestampNanoType/TimestamptzNanoType. A datetime partition value for a nano-precision timestamp column passes through unconverted instead of being converted to nanoseconds since epoch, the same way TimestampType/TimestamptzType already convert to microseconds.

That unconverted datetime object would reach DataFile.partition, where TimestampNanoWriter calls write_int — an int is expected, not a datetime.

Fixes #3652

Changes

• Register a handler for TimestampNanoType/TimestamptzNanoType in _to_partition_representation, using the existing datetime_to_nanos() from pyiceberg/utils/datetime.py (mirrors the existing micros handler exactly).
• Added test_to_partition_representation_timestamps (8 parametrized cases covering None, int passthrough, and datetime conversion for both the existing micros types and the new nano types) and test_to_partition_representation_unrecognized_type_raises (confirms unrecognized input types raise ValueError for both micros and nanos types — previously nano types silently fell through to the generic PrimitiveType handler, which returns the value unchanged rather than raising).

Test plan

• New tests verified to fail against pre-fix code (git stash the fix in partitioning.py, rerun — 3 of 9 new tests fail as expected) and pass post-fix.
• uv run pytest tests/test_transforms.py: 274 passed (up from 265 pre-fix, the +9 are the new tests); the 17 pre-existing failures (missing pyiceberg-core extension in this environment) are unchanged before/after — confirmed identical failure set on unmodified main.
• uv run pytest tests/table/test_partitioning.py: 19 passed / 11 pre-existing failures (same extension-availability cause), unchanged before/after.
• uv run ruff check / ruff format --check: clean on both changed files.
• uv run mypy pyiceberg/partitioning.py: no errors attributable to this file (pre-existing unrelated errors in other files are due to optional dependency stubs not installed in this environment).