fix(langgraph): avoid rewriting unchanged parent keys

[Alexxigang]

Description

• Fixes a parallel execution bug where a compiled subgraph node can return its entire final state back to the parent graph.
• When that final state includes unchanged parent-owned keys, those keys can collide with sibling parent-node updates in the same superstep and raise InvalidUpdateError.
• Wrap subgraph-as-node execution so the parent only receives keys whose values actually changed relative to the subgraph input.
• Adds sync and async regression tests covering the parent/subgraph parallel update case.
• Closes GitHub Issue Title: InvalidUpdateError when using parallel subgraphs with shared state keys in LangGraph #6446

Issue

• GitHub Issue Title: InvalidUpdateError when using parallel subgraphs with shared state keys in LangGraph #6446

Dependencies

• None

Twitter handle

• Alexxigang