fix: drop events tracked after close()

[Zaimwa9]

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Changes

Follow-up to #94. close() cancels the flush timer and flushes once, but the EventProcessor stayed live, so trackEvent, trackExposureEvent and getExperimentFlag called after close() kept buffering events that nothing would ever send. The processor now marks itself stopped: later events are logged and dropped, while the teardown flush and an explicit flushEvents() still complete.

How did you test this code?

flutter analyze clean, flutter test 165 passing. New cases: events tracked after close() are dropped for all three entry points; the buffer pending at close() is still posted.