bug(lib): Explicitly stating wrong return type of `Promise.then` without arguments results in no error

### 🔎 Search Terms

- silenced Promise resolution type mismatch
- Promise generic explicit no error
- no error when Promise type is specified explicitly
- generic inside optional argument

### 🕗 Version & Regression Information

TS v3.3.3 correctly detects the error, which is the earliest Playground version. All other Playground versions silence the error.

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260312#code/MYewdgzgLgBFCm0BcMAKAnEBbAlheAPNOjmAOYB8MAvGprvgHTqIgA2AbvABQCMAlIygALeGG78A3EA

### 💻 Code

```ts
const test: Promise = Promise.resolve(1).then();
```

### 🙁 Actual behavior

No TypeScript errors.

### 🙂 Expected behavior

I expected a similar error to what TS v3.3.3 yields:
> Type `Promise` is not assignable to type `Promise`. Type `number` is not assignable to type `string`.

### Additional information about the issue

_No response_

Reproduce the reported example in the linked TypeScript Playground, comparing the current behavior with the TypeScript 3.3.3 result described in the issue. Trace the type-checking path for Promise.then called without arguments; done means the explicit Promise assignment reports the Promise mismatch.

Written by the indexing model from the issue text.