fix: root messages inbox bounds

[langsmith-fleet]

Summary

• bound the async root messages inbox using the existing stream max_queue_size
• raise a clear RuntimeError when root-scope message buffering overflows instead of allowing unbounded growth
• apply the same bounded/overflow behavior to the sync root messages inbox for parity
• add targeted async/sync tests for inbox bounds and overflow handling

Fixes Corridor finding: https://app.corridor.dev/projects/310af3f2-00f7-413d-8311-2265c3518990/findings/7b6d6f7c-1668-421f-a67b-2b4f3bc49da5

Testing

• python3 -m py_compile libs/sdk-py/langgraph_sdk/_async/stream.py libs/sdk-py/langgraph_sdk/_sync/stream.py libs/sdk-py/tests/streaming/test_scoped_handles.py libs/sdk-py/tests/streaming/test_sync_projections.py

Note: targeted pytest could not run in the sandbox because pytest was not installed in the available Python environment; uv run --no-sync also could not find pytest without syncing dependencies.

[open-swe]

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