Workflow SDK input validation tightened

The workflow builder SDK was accepting malformed inputs without complaint, allowing incorrect usage to create invalid workflows that would fail in confusing ways downstream. Strict type validation was missing at the workflow() function entry point.

Inputs are now validated before any processing occurs. The id and name parameters must be strings—if they aren't, a clear error message shows exactly what types were received. The options parameter is checked to ensure it isn't being misused to pass nodes or connections directly, since those should be added via the .add() and .to() builder methods.

This matters because clear error messages early in execution save debugging time and prevent broken workflows from propagating. The explicit guidance in each error tells developers (and AI agents using the SDK) the correct usage pattern. In the workflow SDK, improved input validation means fewer broken automations reaching production.