`Object.groupBy` should not return `Partial<Record<string, T>>` or `Partial<Record<number, T>>`

まだ誰も着手していません。

### ⚙ Compilation target

es2022

### ⚙ Library

lib.es2024.object

### Missing / Incorrect Definition

The type definitions for `Object.groupBy` state that it returns a `Partial>`. This is useful if `K` is a union, enum, etc. - the result is is likely `Partial`, and the result is consistent with a `Partial>` literal, and having `Partial` helps prevent mistakes from assuming that every key is present in the resulting record. However, it's unnecessary if `K` is a unrestricted number or string: there's no way that a declared key within the result can have an `undefined` value, and the result is more consistent with a `Record` literal than a `Partial>` literal, and any mistakes are more consistently dealt with using TypeScript's `noUncheckedIndexedAccess` option.

See https://github.com/microsoft/TypeScript/pull/56805#issuecomment-2196526425 and https://github.com/microsoft/TypeScript/pull/56805#issuecomment-2385027658 from the PR that originally added types for `Object.groupBy`.

### Sample Code

```TypeScript
type Employee = { name: string, role: 'ic' | 'manager' };
const employees: Set = new Set();

const byName = Object.groupBy(employees, x => x.name);
for (const [name, nameGroup] of byName) {
// TypeScript gives an error that nameGroup may be undefined,
// but it's always defined
console.log(nameGroup.length);
}
```

### Documentation Link

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/groupBy

Start in the lib.es2024.object declarations and inspect the current Object.groupBy type, then find the corresponding library declaration tests. Check how unrestricted string and number keys differ from union or enum keys, and run the relevant TypeScript tests. Done means the broad key cases no longer produce Partial while union-like keys retain appropriate optionality.

索引モデルが issue の本文から書いたものです。