TypeScript error: Property 'toString' does not exist in has-many-through.inclusion-resolver.ts after upgrading to @loopback/repository@8.0.2

Describe the bug

Steps to reproduce

1. Upgrade @loopback/repository from 6.1.5 to 8.0.2 (other LoopBack packages remain at compatible versions).
2. Build the project with TypeScript strict mode enabled.

Current behavior

TypeScript compilation fails with:
node_modules/@loopback/repository/src/relations/has-many/has-many-through.inclusion-resolver.ts:255:36 - error TS2339: Property 'toString' does not exist on type 'NonNullable<(Target & TargetRelations)[Extract<keyof Target, string>]>'.

255 targetEntity[targetKey]?.toString() ?? targetEntity[targetKey],

node_modules/@loopback/repository/src/relations/has-many/has-many-through.inclusion-resolver.ts:269:35 - error TS2339: Property 'toString' does not exist on type 'NonNullable<(Through & ThroughRelations)[Extract<keyof Through, string>]>'.

269 x => x[throughKeyTo]?.toString() ?? x[throughKeyTo],

### Logs

```shell

```

### Additional information

_No response_

### Reproduction

import {Entity, model, property, hasMany} from '@loopback/repository';  @model() export class Category extends Entity {   @property({id: true})   id: number;    @hasMany(() => Product, {through: {model: () => CategoryProduct}})   products?: Product[]; }  @model() export class Product extends Entity {   @property({id: true})   id: number; }  @model() export class CategoryProduct extends Entity {   @property()   categoryId: number;    @property()   productId: number; }

Open src/relations/has-many/has-many-through.inclusion-resolver.ts and inspect the expressions at lines 255 and 269 that trigger TS2339 under strict mode. Reproduce the upgrade and strict TypeScript build with the Category, Product, and CategoryProduct models; done means the build succeeds without these errors.