docs(stdlib): add Args section to StreamChunkingResult class docstring

Fixes [no_class_args] CI failure — the docs build-and-validate checker
requires __init__ parameters to be documented in the class docstring (not
__init__) per Option C convention.

Assisted-by: Claude Code

Author: planetf1

mellea/stdlib/streaming.py

@@ -36,6 +36,11 @@ class StreamChunkingResult:
     Instances are created by :func:`stream_with_chunking`; do not instantiate
     directly.
 
+    Args:
+        mot: The :class:`~mellea.core.base.ModelOutputThunk` from the backend
+            generation call.
+        ctx: The generation context returned alongside the MOT.
+
     Attributes:
         completed: ``False`` if the stream exited early because a requirement
             returned ``"fail"`` during streaming; ``True`` otherwise.