Add JSON single-value serialization for timestamp_ns and timestamptz_ns

[takayoshi-makabe]

Rationale for this change

TimestampNanoType and TimestamptzNanoType are registered for byte conversion but not for to_json / from_json, so the JSON single-value serialization path raises for them. A nanosecond timestamp field with a default cannot even be constructed:

from pyiceberg.types import NestedField, TimestampNanoType

NestedField(1, "ts", TimestampNanoType(), required=False,
            initial_default=1510871468123456789)

TypeError: Cannot deserialize bytes, type timestamp_ns not supported: 1510871468123456789

NestedField runs from_json on initial-default / write-default during validation, so this fails on construction as well as on model_dump_json() and model_validate_json().

Implementation notes

Per Appendix D both types serialize as ISO-8601 strings with nanosecond precision, matching Java's SingleValueParser:

timestamp_ns    "2017-11-16T22:31:08.123456789"
timestamptz_ns  "2017-11-16T22:31:08.123456789+00:00"

from_json returns an int (epoch nanoseconds) rather than a datetime, because Python's datetime only holds microseconds and would silently drop the last three digits. from_bytes already returns an int for these two types. to_human_timestamp_ns / to_human_timestamptz_ns are new — no existing helper formats at nanosecond precision.

Two things left out of scope: the TypeError above says "Cannot deserialize bytes" even from to_json, a pre-existing copy of from_bytes's message that #3856 fixes separately. And this is independent of #3853, which refactors the *_to_nanos parsers while this adds the reverse direction plus the JSON dispatch — they touch different functions.

Are these changes tested?

Yes.

• tests/test_conversions.py: both types added to test_json_single_serialization (int and datetime inputs) and test_json_serialize_roundtrip.
• tests/utils/test_datetime.py: the new formatters, covering the epoch, zero-padded sub-second digits, and a pre-epoch value — each asserted to round-trip back through timestamp_to_nanos / timestamptz_to_nanos.
• tests/test_types.py: end-to-end NestedField round-trip for initial-default / write-default, which is the path in the report above.

Are there any user-facing changes?

Yes. Nanosecond timestamp fields with an initial-default or write-default now serialize and deserialize instead of raising TypeError.