fix(langchain): preserve LangGraph control flow traces #6042

Job	Run time
Unit tests on Python 3.14	34s
linting	18s
Unit tests on Python 3.13	23s
E2E shard 2 tests on Python 3.13	2m 50s
E2E shard 1 tests on Python 3.13	2m 23s
E2E live-provider tests on Python 3.13	2m 2s
Unit tests on Python 3.10	26s
Unit tests on Python 3.11	23s
type-checking	28s
Unit tests on Python 3.12	28s
all-tests-passed	4s
	10m 19s

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