* fix(gateway): stop persisting Overwrite wrappers into empty reducer channels on branch
Thread branching (and POST /state on a never-written channel) wraps copied
reducer values in Overwrite. Upstream BinaryOperatorAggregate.update seeds
an empty (MISSING) channel with values[0] verbatim without unwrapping, so
Union-typed channels (sandbox/goal/todos/promoted) stored the wrapper
literally and the next consumer crashed with TypeError: 'Overwrite' object
is not subscriptable (#4380). Patch the channel to unwrap the first write
(mirroring DeltaChannel semantics), and stop copying thread-scoped channels
(sandbox, thread_data) into branches: the parent's sandbox_id would bind
the branch to the parent's workspace and release lifecycle.
* refactor(checkpoint): drop private _get_overwrite import for a local Overwrite check
Importing langgraph's underscored _get_overwrite at module top level meant an
upstream refactor that drops it - plausibly the same release that fixes the
bug - would fail this module's import and crash startup before the probe can
stand the patch down. Replace it with a local helper on the public Overwrite
type, and fix two test docstring nits.
* refactor(checkpoint): write patch flags via their constants to avoid drift
Both saver patches read their "already patched" idempotence flag through a
module constant (_PATCH_FLAG / _BINOP_PATCH_FLAG) but wrote it as a hard-coded
attribute literal, so renaming the constant would silently break the guard and
double-apply the patch. Write via the same constant (setattr), dropping the
now-unneeded attr-defined ignores.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
