fix: prevent TS2589 type instantiation error in ToolCallback

Use [T] extends [U] pattern on the AnySchema check to prevent
distributive conditional types. This fixes the "Type instantiation
is excessively deep and possibly infinite" error that occurs when
using registerTool after upgrading to v1.23.0.

The issue was that AnySchema (z3.ZodTypeAny | z4.$ZodType) as a union
caused TypeScript to distribute the conditional type check, leading
to exponential type instantiation.

Fixes #1180

Author: felixweinberger

src/server/mcp.ts

@@ -1088,7 +1088,7 @@ export class ResourceTemplate {
  */
 export type ToolCallback<Args extends undefined | ZodRawShapeCompat | AnySchema = undefined> = Args extends ZodRawShapeCompat
     ? (args: ShapeOutput<Args>, extra: RequestHandlerExtra<ServerRequest, ServerNotification>) => CallToolResult | Promise<CallToolResult>
-    : Args extends AnySchema
+    : [Args] extends [AnySchema]
       ? (
             args: SchemaOutput<Args>,
             extra: RequestHandlerExtra<ServerRequest, ServerNotification>