Type parameter leak caused by `this` and reverse mapped type

### 🔎 Search Terms

type parameter leak reverse mapped type type variable this

### 🕗 Version & Regression Information

- This is the behavior in every version I tried

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20251030#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwxAGcMAeAFXhAA9DVgj4AlcHGYUkmLVAcwBp4aANaocAd1QA+KQAocAIwBWALngBvAFDx4AbQDS8HvGEgAnjkTxyAXTWyAlPAC8U6wZsBuTQF8Ha8m9NMDwSeEUlFwJiDFktHQB6BJ14AD0Afm14KEcNLJ04DGQYfAAGbx0ffiyFXPiUwuL8DAALLCIAOhyHCvgq3x7NTSA

### 💻 Code

```ts
declare function test>(obj: {
[K in keyof T]: () => T[K];
}): T;

const obj = test({
// ^? const obj: { a: number; b: T[string]; }
a() {
return 0;
},
b() {
return this.a();
},
});
```

### 🙁 Actual behavior

`obj`'s type refers to `T[string]`

### 🙂 Expected behavior

`T` definitely shouldn't leak into `obj`'s type

### Additional information about the issue

_No response_

使用链接的 TypeScript Playground 和提供的 reverse-mapped type 示例复现该问题。跟踪对象方法使用 `this` 时 checker 如何推断返回类型，然后验证得到的 `obj` 类型不再暴露泛型参数 `T`。

由索引模型根据 Issue 内容生成。