SpanImpl.__exit__ logs GeneratorExit/CancelledError as span errors, causing false failures on every ADK sub-agent transfer

[chen2739]

SpanImpl.__exit__ (braintrust/logger.py:4919-4923) logs any non-None exc_type as a span error:

def __exit__(self, exc_type, exc_value, tb) -> None:
    try:
        if exc_type is not None:
            self.log_internal(dict(error=stringify_exception(exc_type, exc_value, tb)))
    ...

This doesn't distinguish real exceptions from BaseException-only control-flow signals like GeneratorExit and asyncio.CancelledError, which represent normal cleanup, not failures.

This fires reliably when tracing google-adk (2.4.0) agents with sub_agents. ADK's node scheduler (google/adk/workflow/_llm_agent_wrapper.py:381-409) wraps every agent run in async with aclosing(run_method) as run_iter: and does:

if event.actions.transfer_to_agent:
    ...
    transferred = True
    break   # closes run_iter via aclosing.__aexit__ -> throws GeneratorExit

The GeneratorExit propagates through the integration's own aclosing wrappers in braintrust/integrations/adk/tracing.py (_agent_run_async_wrapper, _flow_run_async_wrapper, _flow_call_llm_async_wrapper), each suspended at yield event inside a with start_span(...) block. That with block's __exit__ is SpanImpl.__exit__, which then logs the GeneratorExit as an error.

Impact: any orchestrator agent with sub_agents gets a false error logged on essentially every delegation step, since transfer_to_agent triggers this on every hand-off. A single-agent setup with no sub_agents never hits the transfer_to_agent branch, so the bug is easy to miss until you have a multi-agent workflow.

Suggested fix: in SpanImpl.__exit__, only treat it as an error when exc_type is a real Exception (or explicitly exclude GeneratorExit / asyncio.CancelledError):

if exc_type is not None and issubclass(exc_type, Exception):
    self.log_internal(dict(error=stringify_exception(exc_type, exc_value, tb)))

Repro: an ADK LlmAgent with one or more sub_agents, instrumented via braintrust.wrappers.adk.setup_adk, run through InMemoryRunner. Every transfer_to_agent delegation logs a bogus error on the corresponding span.

Versions: braintrust==0.30.1, google-adk==2.4.0