fix: format identity partition paths for nanosecond timestamps

[anxkhn]

Rationale for this change

An identity transform on a timestamp_ns / timestamptz_ns column renders the
raw integer value in its human string, so identity partition directory paths
come out as ts=1740342104375612001 instead of an ISO-8601 timestamp. The
microsecond timestamp / timestamptz types render correctly (e.g.
ts=2025-02-23T20:21:44.375612), so this is an inconsistency in the nanosecond
timestamp types added for the v3 spec.

The cause is in _int_to_human_string (pyiceberg/transforms.py): it dispatches
DateType, TimeType, TimestampType, and TimestamptzType, but has no
registration for TimestampNanoType / TimestamptzNanoType, so those values
fall through to the base handler str(value). Because
IdentityTransform.to_human_string and PartitionSpec.partition_to_path
(pyiceberg/partitioning.py) both route through _int_to_human_string, an
identity-partitioned nanosecond timestamp column produces a raw-integer
partition path.

This change adds to_human_timestamp_ns / to_human_timestamptz_ns helpers in
pyiceberg/utils/datetime.py and registers them for the two nanosecond
timestamp types. Python's datetime only supports microsecond precision, so the
helpers render the microsecond instant at a fixed 6 fractional digits and append
the remaining 3 sub-microsecond digits, yielding a full 9 fractional-digit
ISO-8601 string with no trailing-zero trimming. This matches the Java
DateTimeUtil.nanosToIsoTimestamp / nanosToIsoTimestamptz behaviour used by
Transforms.identity().toHumanString() (for example
1510871468000001001 -> 2017-11-16T22:31:08.000001001, with the zone offset kept
after the fraction for the tz variant).

Are these changes tested?

Yes.

• tests/utils/test_datetime.py: new test_to_human_timestamp_ns and
  test_to_human_timestamptz_ns cover the epoch boundary and the
  sub-microsecond digit boundaries (0, 1, 999, 1000), a whole-second-plus-1ns
  case (where the microsecond fraction is zero), and the Java reference value.
• tests/test_transforms.py::test_identity_human_string: adds timestamp_ns
  and timestamptz_ns rows alongside the existing type rows.
• tests/table/test_partitioning.py::test_partition_spec_to_path_timestamp: an
  end-to-end PartitionSpec.partition_to_path check for the microsecond and
  nanosecond timestamp/timestamptz types (the user-facing symptom).

The targeted subset passes (14 passed). Running the full tests/test_transforms.py,
tests/utils/test_datetime.py, and tests/table/test_partitioning.py files
shows only the pre-existing failures that require the optional pyiceberg_core
extension; these new tests do not touch that path. Integration tests
(Docker + Spark) were not run in this environment; the behaviour is covered by
the unit tests end-to-end through partition_to_path.

Are there any user-facing changes?

Yes. Identity partition paths for timestamp_ns and timestamptz_ns columns are
now ISO-8601 strings (matching the microsecond timestamp types and the Java
implementation) instead of raw integers. IdentityTransform.to_human_string for
these types changes correspondingly.