refactor(types): remove all any from extend-adapter type utilities

Replace any-typed function shapes with a sound AnyAdapterFactory
constraint built on never params / unknown return (parameters are
contravariant, so never accepts every factory). The deliberate
model-union widening now lives in an overload signature instead of an
unchecked assignment, so no cast or any remains anywhere in the file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tombeckenham

packages/ai/src/extend-adapter.ts

@@ -144,15 +144,23 @@ type ExtractCustomModelNames<TDefs extends ReadonlyArray<ExtendedModelDef>> =
 // Factory Type Inference
 // ===========================
 
+/**
+ * The widest factory shape `extendAdapter` accepts: any function taking a
+ * model as its first parameter. Parameters are contravariant, so `never`
+ * params and an `unknown` return accept every factory without resorting
+ * to `any`.
+ */
+type AnyAdapterFactory = (model: never, ...args: Array<never>) => unknown
+
 /**
  * Infer the model parameter type from an adapter factory function.
  * For generic functions like `<T extends Union>(model: T)`, this gets `T` which
  * TypeScript treats as the constraint union when used in parameter position.
  */
 type InferFactoryModels<TFactory> = TFactory extends (
   model: infer TModel,
-  ...args: Array<any>
-) => any
+  ...args: Array<never>
+) => unknown
   ? TModel extends string
     ? TModel
     : string
@@ -162,7 +170,7 @@ type InferFactoryModels<TFactory> = TFactory extends (
  * Infer the adapter return type from a factory function.
  */
 type InferAdapterReturn<TFactory> = TFactory extends (
-  ...args: Array<any>
+  ...args: Array<never>
 ) => infer TReturn
   ? TReturn
   : never
@@ -173,16 +181,16 @@ type InferAdapterReturn<TFactory> = TFactory extends (
  * Note: overloaded factories resolve against their last overload (a
  * `Parameters` limitation).
  */
-type InferRestArgs<TFactory extends (...args: Array<any>) => any> =
-  Parameters<TFactory> extends [any?, ...infer TRest] ? TRest : []
+type InferRestArgs<TFactory extends AnyAdapterFactory> =
+  Parameters<TFactory> extends [unknown?, ...infer TRest] ? TRest : []
 
 /**
  * The factory signature produced by `extendAdapter`: accepts both original
  * and custom model names while preserving all remaining parameters and the
  * return type of the original factory.
  */
 type ExtendedFactory<
-  TFactory extends (model: any, ...args: Array<any>) => any,
+  TFactory extends AnyAdapterFactory,
   TDefs extends ReadonlyArray<ExtendedModelDef>,
 > = (
   model: InferFactoryModels<TFactory> | ExtractCustomModelNames<TDefs>,
@@ -237,9 +245,15 @@ type ExtendedFactory<
  * ```
  */
 export function extendAdapter<
-  TFactory extends (model: any, ...args: Array<any>) => any,
+  TFactory extends AnyAdapterFactory,
   const TDefs extends ReadonlyArray<ExtendedModelDef>,
->(factory: TFactory, _customModels: TDefs): ExtendedFactory<TFactory, TDefs> {
+>(factory: TFactory, _customModels: TDefs): ExtendedFactory<TFactory, TDefs>
+// The implementation signature stays at the honest `AnyAdapterFactory` width;
+// the overload above performs the deliberate model-union widening.
+export function extendAdapter(
+  factory: AnyAdapterFactory,
+  _customModels: ReadonlyArray<ExtendedModelDef>,
+): AnyAdapterFactory {
   // At runtime, we simply pass through to the original factory.
   // The _customModels parameter is only used for type inference.
   // No runtime validation - users are trusted to pass valid model names.