`asyncio.gather` propagates the first failure while its siblings run on, and the
caller unwinding from that closes the set through `async with` — so a sibling
still reading reads through a closed session. Seven fan-outs were affected:
lookup, search, image enrichment, multimodal picture loading, context expansion,
document listing and counting, and the sandbox's document load.
`gather_all` cancels and drains the rest, then re-raises the original exception.
A `TaskGroup` would drain them too but raise an `ExceptionGroup`, which every
caller and both CLIs' exception handlers would have to unwrap. The two
`return_exceptions=True` gathers in session opening and teardown already drain
their children and are left alone.
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