fix(langgraph): Fix chat model streaming for streamMode messages #2061

Job	Run time
Build	1m 59s
check-readmes-synced	3s
Check formatting	1m 24s
Check linting	1m 43s
Validate Deno and Node dependencies in sync	5s
validate-new-notebooks	4s
Unit Tests (ubuntu-latest, 18.x)	1m 34s
Unit Tests (ubuntu-latest, 20.x)	1m 27s
Unit Tests (ubuntu-latest, 22.4.1)	2m 11s
Unit Tests (windows-latest, 22.4.1)	6m 42s
Unit Tests (macos-latest, 22.4.1)	4m 11s
	21m 23s

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