Fix LLMObs span double-exit issue with exit state tracking

[Copilot]

The LLMObs span cleanup logic could call __exit__ twice on the same span—once in the error handler and once in the success path—causing potential errors.

Changes

• Add llmobs_span_exited flag to track span exit state
• Set flag after exiting span in error handler
• Check flag before exiting span in success path

llmobs_span = None
llmobs_span_exited = False
# ... span setup ...

try:
    response = await model_request(...)
except Exception as e:
    if llmobs_span:
        self._annotate_llmobs(llmobs_span, llmobs_input, error=e)
        llmobs_span_exited = True
    raise

if llmobs_span and not llmobs_span_exited:
    self._annotate_llmobs(llmobs_span, llmobs_input, response=response)

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