Allow step end_time overwrite

[gabrielBarbutti]

Minor fix: Preserve manually set end_time on Step.end()

Context

When using the with syntax to log a step, we sometimes manually set start_time and end_time (e.g. to match historical timestamps or upsert an existing thread).
However, the current implementation of Step.end() always overwrites end_time with utc_now(), which can lead to inaccurate timing data.

Change

Updated the end() method to preserve the manually-set end_time if it already exists:

self.end_time = utc_now() if self.end_time is None else self.end_time

Example of end_time overwriting

async with self.client.step(
    id=self.generate_uuid_v5_from_id(run.id),
    thread_id=self.generate_uuid_v5_from_id(thread_id),
    parent_id=self.generate_uuid_v5_from_id(parent_id),
    type="run",
    name=assistant_name,
) as step:
    step.created_at = created_at
    step.start_time = created_at
    step.end_time = completed_at
    step.input = input
    step.output = output
    step.generation = generation

Now correctly logs completed_at instead of the current time.

Why it matters

This ensures accurate time tracking when steps are being reconstructed or backfilled, and avoids unintentionally overwriting data.