Performance [P3]: Serialize history events in one pass

[berndverst]

What is the issue?

History event export deep-copies the dataclass graph and then recursively walks it again to convert values for JSON.

What is the impact?

Exporting large orchestration histories performs avoidable allocations and CPU work. This is limited to the history export feature but can be material for large archive jobs.

Details about the issue including code reference

Relevant code:

durabletask-python/durabletask/history.py

Lines 44 to 45 in 55d8e0b

	def to_dict(self) -> dict[str, Any]:
	return _to_serializable(asdict(self))

durabletask-python/durabletask/history.py

Lines 329 to 339 in 55d8e0b

	def _to_serializable(value: Any) -> Any:
	if isinstance(value, datetime):
	return value.isoformat()
	if isinstance(value, list):
	return [_to_serializable(item) for item in cast(list[Any], value)]
	if isinstance(value, dict):
	return {
	key: _to_serializable(item)
	for key, item in cast(dict[Any, Any], value).items()
	}
	return value

durabletask-python/durabletask/extensions/history_export/serialization.py

Lines 42 to 52 in 55d8e0b

	def event_to_dict(event: history.HistoryEvent) -> dict[str, Any]:
	"""Convert a :class:`history.HistoryEvent` into a JSON-safe dict.
	A discriminator field ``event_type`` is added so downstream
	consumers can distinguish event subclasses without inspecting
	their fields.
	"""
	payload: dict[str, Any] = event.to_dict()
	# Insert the discriminator first so the resulting dict orders it
	# near the front of the JSON object even before any sorting.
	return {"event_type": type(event).__name__, **payload}

HistoryEvent.to_dict() calls _to_serializable(asdict(self)). dataclasses.asdict() recursively deep-copies nested dataclasses, lists, and dictionaries; _to_serializable() then walks lists and dictionaries again. History export invokes this path through event_to_dict().

A potential or proposed solution

Implement a one-pass recursive serializer over dataclass fields, lists, dictionaries, and datetimes instead of calling asdict(). Preserve the existing output shape, datetime format, and handling of nested history dataclasses.