bubus derives the default bus name from the class name, so the
ResilientEventBus default factory changed session bus names from
EventBus_* to ResilientEventBus_*, breaking the EventBus_ prefix contract
asserted in tests/ci/browser/test_session_start.py. Default the subclass
name back to EventBus_<id> when none is given (explicit names still honored).
The V2 worker reuses a keep_alive BrowserSession across warm Lambda
invocations. At the end of a run Agent.close() stops the session's event
bus and nulls out its async primitives (event_queue / _on_idle) to release
the event loop. On resume the worker can step() the bus before any
dispatch() restarts it, and stock bubus EventBus.step() asserts
"EventBus._start() must be called before step()" in that state — crashing
the run deterministically so the task dead-letters after max receives
(~1,600+ occurrences over 2 days).
Wrap the session's bus in a ResilientEventBus subclass whose step() and
wait_until_idle() are safe no-ops when the bus has not been started,
instead of asserting. The nulling stays (it's what lets the next dispatch()
recreate a fresh queue and _start() the bus), so a later dispatch() still
restarts the bus and processes events normally.
Fixes ENG-5280.
Saurav Panda