The streaming-hook dispatcher runs one worker per callback; delivery
order is FIFO per hook, never across hooks. Two tests pinned a global
start->delta->delta->end interleaving that three concurrent workers
don't guarantee, flaking CI twice within an hour of #84924 landing.
Also wait for the full event count before shutdown so late deltas
aren't dropped mid-assert.
Salvage of PR #64317 (@deaneeth) onto current main, implementing #64161:
observer-only on_stream_start / on_stream_delta / on_stream_end /
on_interim_message plugin hooks dispatched through a host-owned bounded
queue (one worker per callback) so plugin callbacks never run inline on
the token path. Reasoning deltas are opt-in via
plugins.stream_reasoning_deltas.
Teknium