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Christian Bromann e3a1933a88 fix(core): keep stream() on 3-tuple shape (#2489 )

## Summary
- Fixes `ValueError: too many values to unpack (expected 3)` when JS
graphs on `@langchain/langgraph` >=1.3 stream through the legacy path
(plain `stream()` / `streamEvents` v2 → `on_chain_stream`), which
affected deepagents 1.10.x and similar stacks on runtimes that expect
Python's 3-tuple stream shape.
- **Root cause:** `_streamIterator` always yielded a 4-element
`[namespace, mode, payload, meta]` when `subgraphs` + multi-mode and a
checkpointer were active, even though `StreamChunkMeta` is only for the
native v3 protocol stream (`streamEvents(..., { version: "v3" })` /
`pump()`).
- **Fix:** Introduce `isV3` and only append `meta` when `options.version
=== "v3"`; all other consumers get the Python-aligned 3-tuple
`[namespace, mode, payload]`.
- **Tests:** Add `stream() shape parity with Python` regression coverage
(3-tuples for subgraphs + multi-mode with checkpointer; v3 still emits
companion `checkpoints` events).

2026-06-04 12:59:18 -07:00