When a generator exits with wasAborted=true, the AbortController remains in
aborted state but generatorPromise is set to null. When a new observation
arrives, ensureGeneratorRunning() sees generatorPromise=null and tries to
start a new generator, but the new generator immediately sees
signal.aborted=true and exits, causing an infinite "Generator aborted" loop.
This fix resets the AbortController if it's already aborted before starting
a new generator, allowing the session to recover from the stuck state.
Bug reproduction:
1. Session receives observations
2. Something causes the generator to be aborted
3. generatorPromise = null, but abortController.signal.aborted = true
4. New observation arrives → starts generator → immediately aborted → loop
Fix: Check if abortController.signal.aborted before starting generator,
and create a new AbortController if needed.
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