Add a zodState() helper that attaches a lazy `~standard.jsonSchema.input`
to a zod schema so LangGraph's StateSchema.getJsonSchema() emits the
field into the graph's output_schema. Without it, zod v4 fields carry
`~standard.validate` + `vendor` only, `isStandardJSONSchema` returns
false, and the field is silently dropped from output_schema — which in
turn causes the AG-UI LangGraphAgent proxy to filter the value out of
STATE_SNAPSHOT events on the wire, so the frontend never sees it even
though the underlying thread state has the data.
Apply zodState to the middleware's own `copilotkit` state field so it
surfaces in output_schema and export it for demos to use on custom
state fields (todos, documents, etc.).
Uses `z.toJSONSchema` when available (zod v4 subpath) and falls back to
an empty object, which is sufficient to make langgraph-api include the
key in output_schema.
Benjamin Taylor