Function fallback

fallback(
executors: readonly ExecutorInput[],
options?: FallbackOptions,
): Executor
Tries executors in order until one succeeds.

A streaming attempt that has already emitted a chunk is never abandoned.
Parameters
- executors: readonly ExecutorInput[]
  Executors or model ids to try, best first.
- options: FallbackOptions = {}
  Fallback predicates and hooks.
Returns Executor
An executor that fails over.
Throws
Error when executors is empty.
Example
```
const upstream = fallback([
  retry("default/glm-5.2", { attempts: 3 }),
  "fast/gemma-4-31b",
]);
```