kwargs is the event payload — an opaque JSON object whose shape depends on
name and on the emitting SDK. For example LangChain emits {"token": {...}}
for new_token events, tool-call start/end details for tool events, and
arbitrary user-defined payloads for custom events. Clients should treat kwargs
as untyped JSON: do not assume specific keys exist for a given name, and
tolerate additional unknown keys appearing over time.
