Discriminated union with 11+ items does not handle nullability same as union with <10 items

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### 🔎 Search Terms

10 11 union undefined type
Nullability narrowing is inconsistent between type discriminated unions of <=10 items and >10 items

### 🕗 Version & Regression Information

- This changed between versions 4.2.3 and 4.3.5 (before it would complain about undefined in both cases)

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.2#code/PTAEFUDsEsHtNAd2gFwBagIyYNSlQKYC2AzqACawFmSwqhoCGk5ANgaJAK6uuMBG0VqgCeoEoyIdGZLjHhJUGADyYADPhTESAKHIEAxnwBOHdvUYBmAFygA3qBQiADgVsAiTO9ABfUAB97Rxc3UHcAJm8-QLl9ADNoSAJyAG4daDiACisAQgA6J1dQAF5SsK8ASjsdUFrQEFAAcitG-DJnWBISaH5WMUbYggSk8ka8zMwADjUAFkmKmrqDeBJYdjzWWABzbMsCkIq0nx09QxMzAgt1WwdC0M8ogKC7j0jfJ9uQj0tHwM-XDwzX7PL5hACswP+9wAbJDggCwgB2OEvMKTFGg9wATgxCM8amBg2GyTSGWy6nydxKZQeVUWtQaADlYKACMZjLBjKBMvxLloubymAA3ODGAD8CzqoGWkFW602O0Y6n2rkOOh8QA

### 💻 Code

```ts
// Union with 11+ items does not handle nullability same as union with <10 items
declare let a3: { type: "1" } | { type: "2" } | undefined;
if(a3!.type === "1"){
// 'a3' is possibly 'undefined'.(18048)
console.log(a3.type);
}

declare let a10: { type: "1" } | { type: "2" } | { type: "3" } | { type: "4" } | { type: "5" } | { type: "6" } | { type: "7" } | { type: "8" } | { type: "9" } | { type: "10" } | undefined;
if(a10!.type === "1"){
// No error (better behavior?)
console.log(a10.type);
}
```

### 🙁 Actual behavior

Complains about undefined for unions with >10 members, but not for unions <11

### 🙂 Expected behavior

It seems like it should not error in either case - the code inside the `if` can only execute if the `a3` variable is not undefined

### Additional information about the issue

Relevant function in checker.ts: getKeyPropertyName(). That is where the 10 magic value comes from.

Inizia con la riproduzione fornita in Playground ed esamina checker.ts, in particolare getKeyPropertyName(), dove viene identificata la soglia di 10 elementi. Confronta il narrowing della nullabilità per le due dimensioni dell’unione e considera il comportamento previsto indicato nell’issue: nessuno dei due esempi dovrebbe segnalare undefined all’interno del blocco protetto.

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