fix(langgraph): unwrap an Overwrite first write on an empty BinaryOperatorAggregate channel

[哈基米 (BlueX888)]

Fixes #9017

BinaryOperatorAggregate.update now runs the first write through _get_overwrite when the channel is empty, so an Overwrite first write stores its payload instead of the wrapper — matching the already-seeded behavior of the same method and DeltaChannel.update. The seeded seen_overwrite flag also means a second Overwrite in the same update still raises, and plain values after a leading Overwrite are skipped exactly as they are on a seeded channel.

Verified with three new unit tests in tests/test_channels.py (first-write unwrap, later plain values skipped, two overwrites raise), plus the full pytest tests/ suite passing except the redis/remote cases that also fail on main in this environment without those services.

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