fix(langgraph): remove shadowed stream_writer in astream and add async tests

[dumko2001]

Description:
While investigating #6447, I identified a shadowed variable definition and a gap in test coverage:

1. Shadowed Variable: The stream_writer variable was defined in Pregel.astream (lines ~2838) but was immediately potentially shadowed/ignored by subsequent logic. This is a potential source of ambiguity in async context propagation. This PR removes the redundant definition.
2. Missing Test Coverage: There were no existing tests explicitly verifying get_stream_writer() works correctly when used inside a ToolNode running async tools. I added libs/langgraph/tests/test_get_stream_writer_async.py to verify this behavior and prevent regressions.

Issue: Addresses #6447

Dependencies: None

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