Set new spans added by on_trace_start as current/unset them

[CLowbrow]

No description provided.

[ibolmo]

would be great to see a real example repro the problem and that with this fix fixes the problem.

[dpdiliberto]

To repro, I used the following:

from agents import set_trace_processors, trace

from braintrust import flush, init_logger, start_span
from braintrust.wrappers.openai import BraintrustTracingProcessor


def run_demo():
    bt_logger = init_logger(project="8375-agents-sdk")

    set_trace_processors([BraintrustTracingProcessor(bt_logger)])

    with trace(workflow_name="workflow"):
        with start_span(name="child-span"):
            pass

    flush()


if __name__ == "__main__":
    run_demo()

Before the fix, I got the workflow root span and child-span in separate traces.

After this fix and after running

pip install --force-reinstall \ "braintrust[openai-agents] @ git+https://github.com/braintrustdata/braintrust-sdk.git@alex/set-correct-parents#subdirectory=py"

I got the child-span span within the workflow trace.