refactor(dfir_pipes): Add an `fn is_terminated(&self) -> bool` method to `FusedPush`, use in `SendPush` and `SendSink` instead of the `pull_ended: bool` field.

[MingweiSamuel]

refactor(dfir_pipes): Add an fn is_terminated(&self) -> bool method to FusedPush, use in SendPush and SendSink instead of the pull_ended: bool field.

Originally posted by @MingweiSamuel in #2644 (comment)