Add event triggering for streaming

I have uploaded a patch which uses a suggested fix in a AI module.

Each AI provider supporting stream should be able to iterate any way it needs to, so I suggest this change: https://git.drupalcode.org/issue/ai-3493277/-/blob/3493277-extend-loggin...

This way the getIterator() works as a wrapper template, ensuring compatibility, which at the end of stream triggers the event. If a provider requires different implementation if iterator, it can override overrideGetIterator().