config: pass the Ray placement group as a launch argument

`RayEngine.__init__` set the caller's `PlacementGroup` on `ServerArgs` as an
undeclared attribute, and every consumer read it back off the config object:
the non-DP launch path took `pg` and `is_custom_pg` from it, the DP path had to
re-attach the handle after `dataclasses.replace` dropped it, the DP controller
asked the config whether the group was the caller's, and the Ray HTTP launcher
had to clear the field so a stale handle could not leak in.

A live cluster object is not config. It now travels as a `placement_group`
argument on the two launch hooks — `Engine._launch_subprocesses` and
`Engine._launch_scheduler_processes` — defaulting to `None`, with `RayEngine`
holding the caller's group in `_placement_group` and the DP path passing
`is_custom_pg` down to the controller. The public API is unchanged:
`RayEngine(placement_group=pg, ...)` still works, `pg` still falls back to the
ambient group, and the HTTP launcher gets `None` from the default.

Writer ratchet 18 -> 15.

test/registered/unit/test_server_args_writer_ratchet.py

@@ -49,7 +49,7 @@ _EXCLUDED = (
     "multimodal_gen",
 )
 
-_BASELINE = 18
+_BASELINE = 15
 
 
 class TestServerArgsWriterRatchet(CustomTestCase):