Conditional type flow types missing in the template in mapped types with as clause

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### 🔎 Search Terms

as clause renaming filtering mapped type conditional type flow

### 🕗 Version & Regression Information

- This is the behavior in every version I tried

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250222#code/C4TwDgpgBAggxnCZgDkD2A7FBXANrgQwCNcIAeAFSggA9gIMATAZygG8BfAPigF4oKAbgBQw0JAERmwSj35thUKAG0A0lACWGKAGsIINADMBUAqwpqAutToMW7DlAD8UdQC4oGCADcIAJ0sPeERkdCw8QhJyC1VLLhEOESA

### 💻 Code

```ts
type AcceptNonNullable = T;

type Test = {
[K in keyof T as T[K] extends {} ? K : never]: AcceptNonNullable;
};
```

### 🙁 Actual behavior

`AcceptNonNullable` errors with:
```
Type 'T[K]' does not satisfy the constraint '{}'.
Type 'T[keyof T]' is not assignable to type '{}'.
Type 'T[string] | T[number] | T[symbol]' is not assignable to type '{}'.
Type 'T[string]' is not assignable to type '{}'.(2344)
```

### 🙂 Expected behavior

It doesn't have to error. It's known that `T[K]` has to extend `{}` at this position

### Additional information about the issue

_No response_

Start with the linked TypeScript Playground reproduction and inspect how the type checker handles the mapped type's `as` clause and conditional constraint. Trace why the constraint on `AcceptNonNullable` is not recognized, then confirm the example no longer reports error 2344 while preserving the filtering behavior.

Written by the indexing model from the issue text.