Multiple keys being remapped to same key causes unstable modifier propagation

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### 🔎 Search Terms

Mapped Types, Key Remapping

### 🕗 Version & Regression Information

- This has been happening since first release of key remapping at 4.1 (tested with v4.1.5 at Playground) until nightly (tested with v6.0.0-dev.20250822 at Playground).

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAShC2BDMBpCIAqB7AkgOwEtgDEAbABUQCdgAeDAPigF4oBvAKCm6gG0UoBPFADW6LADMoGKIgDOUARAAewCHgAmCgAYASNkIkQqUAGIEqc4AF8AdPsPGoAfThXr2qAH4zFq1AAuRQBdILwAV1JSAG4Oa1iOUEhfS2AAeTBiLDwyFlgEZDRMXEJiMkoaWk4eKAB6WqgAPS8uHgAiCSwsWwAjajavIKsqIQBzWJqOrt7EAC82sPD4HuNY6wYEpOhzVLgAR3CLCA08uCRUdGx8IhIKajpqnnqmlsnO7r75xeXV1u4pj79QZQYZjNYbDhAA

### 💻 Code

```ts
type RemapKeyToInitialPart = {
[K in keyof T as K extends `${infer First}.${infer _Rest}` ? First : K]: null;
};

type FirstOptional = RemapKeyToInitialPart<{
// ^?
"foo.bar"?: string;
"foo.baz": number;
}>;

type FirstRequired = RemapKeyToInitialPart<{
// ^?
"foo.baz": number;
"foo.bar"?: string;
}>;
```

### 🙁 Actual behavior

`type FirstOptional` and `type FirstRequired` results in different types:
- `type FirstOptional` has *optional* property `foo`
- `type FirstRequired` has *required* property `foo`

### 🙂 Expected behavior

`type FirstOptional` and `type FirstRequired` results in same type. I would expect it to be same as ~~`type FirstRequired`~~ `type FirstOptional`.

### Additional information about the issue

My motivation was to implement a utility `type Expand` which transforms `{ "foo.bar": number }` to `{ foo: { bar: number } }` ([playground](https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true#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-IXe01iiWE6XJ62K5Wq9MaqDa3WAg0QI3AHPx0OWhK8eUlu1Rx184hAA))

Riproduci i due esempi di tipi mappati usando il TypeScript Playground collegato e confronta FirstOptional con FirstRequired. Analizza il comportamento del compilatore nel rimappamento delle chiavi, quindi aggiungi un test di regressione che dimostri che entrambi gli ordini delle dichiarazioni producono la stessa opzionalità delle proprietà; il lavoro è completato quando il test passa con risultati identici.

Scritto dal modello di indicizzazione a partire dal testo della issue.