Type the prompt wrapper's tensor slice for open-ended ranges

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/worker/prompt.ts

@@ -49,7 +49,7 @@ export interface PromptTokenizer {
 }
 
 interface SliceableTensor {
-  slice(...slices: (number[] | null)[]): unknown;
+  slice(...slices: ((number | null)[] | null)[]): unknown;
 }
 
 const WRAPPED = Symbol("whisper-web-prompt-support");