fix(langchain): remove _privateState

[christian-bromann]

In preparation to implement ToolRuntime and AgentRuntime I fixed how we derive state between middleware nodes and removed the _privateState key from the agent which we used before to store information about tool and model counts. With current middleware primitives this isn't needed anymore.

The middleware implementation for modelCallLimitMiddleware and toolCallLimitMiddleware now aligns much closer to what we do in Python too.

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