fix(bloc): make isClosed return true immediately when close is called

[ersanKolay]

Summary

• Fixes a race condition where isClosed returned false during Bloc.close() teardown, but add() would throw a StateError because the internal _eventController was already closed.
• Root cause: isClosed was based on _stateController.isClosed, but Bloc.close() closes _eventController first. This created a window where checking isClosed before add() was unreliable.
• Introduces a _closed flag in BlocBase set at the start of close(), and overrides isClosed in Bloc to also check _eventController.isClosed.
• Internal emit/onEmit checks use _stateController.isClosed directly so in-progress event handlers can still emit states during teardown.

Closes #4749

Test plan

• All 118 existing tests pass
• New test: Bloc.isClosed returns true immediately when close() is called (without await)
• New test: Cubit.isClosed returns true immediately when close() is called (without await)
• dart analyze clean (no errors or warnings)

[felangel]

Thanks for the PR but I’m not sure this is the correct fix. I think the first step here is to make a minimal reproduction sample. The tests you added don’t reflect the issue linked.

[ersanKolay]

Thanks for the feedback. I added a minimal reproduction sample as a comment on #4749 and updated the PR with a test that reproduces the exact scenario from the issue: a Bloc using emit.onEach where onData guards add() with isClosed, but external stream data during close() teardown causes a StateError.

Let me know if you'd like a different approach for the fix itself.

[felangel]

Thanks for the feedback. I added a minimal reproduction sample as a comment on #4749 and updated the PR with a test that reproduces the exact scenario from the issue: a Bloc using emit.onEach where onData guards add() with isClosed, but external stream data during close() teardown causes a StateError.

Let me know if you'd like a different approach for the fix itself.

Thanks I'll take a look shortly! 🙏