Serialize tool calls that rewrite a plain state channel

[naftali-g]

ToolNode runs every tool call of an assistant turn in one superstep. For a tool that rewrites a plain (LastValue) channel, two such calls in one turn are two writes to that channel, and LangGraph aborts the graph with InvalidUpdateError. The model-side lever, parallel_tool_calls=False, is a property of the request, not of a tool, so it serializes everything to protect one.

serial_tools.py serializes at execution time instead:

• a tool declares the plain keys it writes with serialize_writes(tool, "curr_spec"), stored in BaseTool.metadata;
• make_tool_node(tools, **kwargs) returns the stock ToolNode when nothing declares, and wraps it otherwise;
• in a wrapped batch, undeclared calls run concurrently through ToolNode with their updates passed through verbatim, so reducer-backed channels still get one write per call;
• declared calls run one at a time, each seeing the previous one's writes through InjectedState, and their state writes collapse into a single update — one write per superstep, no reducer needed.

Sequencing matches what a model assumes when it batches two edits to one buffer: the second applies to the result of the first. A stale anchor then fails as an ordinary tool rejection it can retry, rather than aborting the graph.