Python: generated parse_rfc3339 rejects timestamps with more than 6 fractional digits #297

What happens

For an OffsetDateTime field, the Python generator emits a parse_rfc3339 validator that tries two datetime.strptime formats ending in %S.%fZ and %SZ. Python's %f accepts at most 6 digits. The time crate's RFC 3339 serializer (#[serde(with = "time::serde::rfc3339")]) writes up to 9 fractional digits, so any timestamp with nanosecond precision fails validation on the Python side.

On Linux, OffsetDateTime::now_utc() has nanosecond precision, so this is the normal case for values produced by a Rust service, not an edge case.

What should happen

A value serialized by the Rust side should validate in the generated Pydantic model. Digits beyond microsecond precision can be dropped, since datetime cannot represent them.

Minimal repro

#[typeshare]
pub struct Foo {
    #[serde(with = "time::serde::rfc3339")]
    pub time: OffsetDateTime,
}

Rust serializes datetime!(2024-05-01 12:34:56.123456789 UTC) as:

{"time":"2024-05-01T12:34:56.123456789Z"}

Feeding that to the generated model (the current core/data/tests/test_serde_iso8601/output.py):

Foo.model_validate_json('{"time":"2024-05-01T12:34:56.123456789Z"}')
# pydantic_core.ValidationError: 1 validation error for Foo
# time
#   Value error, Invalid RFC 3339 date format: 2024-05-01T12:34:56.123456789Z

"2024-05-01T12:34:56.123Z" and "2024-05-01T12:34:56Z" validate fine.

Root cause

The deserialization_content for datetime in json_translation_for_type (core/src/language/python.rs) passes the raw string to strptime with %f, which only matches 1 to 6 digits.