Null is treated as absent at every layer; unknown delta, tool and
function fields merge by OpenAI streaming conventions (strings concat,
label keys idempotent, indexed arrays by index) and conflicts drop the
key and record its path instead of failing the call. The accumulator
now tracks finish and usage-after-finish in both modes, exposes the
reasoning alias, and non-stream parsing splits shape checks from
snapshot construction.
Red: 20 of 47 tests in tests/unit/test_openai_completion.py failed
before this change (e.g. test_null_known_and_unknown_fields_are_absent
raised TransientError 未知 delta 字段; finish_seen/raw_message missing).
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