Create router

Route Evaluation: Routes are evaluated in order based on CEL conditions. The first route whose condition evaluates to true is selected. If no route matches, the defaultRoute is used. If defaultRoute is not set and no conditional routes match, the API returns an error: "No route matched. Configure a default route or adjust conditions."

Variant Selection: Within the selected route, a variant is chosen based on weights. Weights must sum to exactly 100 within each route (they are not normalized). Each route's variants are weighted independently - weights are not shared across routes.

A long-running operation (LRO). Poll until done is true.