[Chat][Platform] Notify stream listeners about deltas yielded from an injected generator

[lazerg]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #2330
License	MIT

When a stream listener replaces a delta with a generator, StreamResult::getContent() forwarded it with yield from. PHP does not re-enter the outer generator for the values of a delegated one, so the deltas coming out of that generator were never sent to onDelta(). The toolbox StreamListener injects the tool-call follow-up exactly that way, which is why ChatStreamListener saw nothing after a tool call and stored an empty assistant message, while the frontend received the text.

The delta dispatch now recurses into an injected generator and notifies the listeners about each delta it yields. The listener that injected the generator is left out of that nested round, so it does not reprocess its own output: PartialObjectStreamListener would otherwise buffer the same text twice and re-inject forever, and the toolbox StreamListener would drop the follow-up it just produced. The order and the number of yielded deltas stay the same.

The forwarding also went through yield from, which keeps the inner generator's keys, so the outer stream repeated key 0 and iterator_to_array() on it silently lost deltas. Yielding one by one gives the whole stream sequential keys again.

[lazerg]

The Examples / PHP 8.5 job failed while pulling the service images, with a 502 from the registry

[lazerg]

Hey @chr-hertel, could you take a look at this fix for notifying stream listeners about deltas yielded from an injected generator? The earlier CI failure was a registry 502, unrelated to the change.