CultureInfo.GetCultureInfo("root") is an accepted but incomplete culture which can raise NullReferenceException on internals

### Description

"und" is the correct BCP-47 tag for the "undetermined language" and maps correctly to `CultureInfo.InvariantCulture` when using `CultureInfo.GetCultureInfo("und")`.

"root" is the alternative moniker used by CLDR.
When using `CultureInfo.GetCultureInfo("root")` it does not throw a `CultureNotFoundException`.
Rather it returns an incomplete unknown culture with LCID 4096.
And if one attempts to read NumberFormat off of it:

```
> System.Globalization.CultureInfo.GetCultureInfo("root").NumberFormat
System.NullReferenceException: Object reference not set to an instance of an object.
+ System.Globalization.NumberFormatInfo.InitializeInvariantAndNegativeSignFlags()
+ System.Globalization.CultureInfo.get_NumberFormat()
```

### Reproduction Steps

Open any C# repl and execute:
`System.Globalization.CultureInfo.GetCultureInfo("root").NumberFormat`

### Expected behavior

Either `CultureInfo.GetCultureInfo("root")` throws a `CultureNotFoundException` or it produces an actual intact culture.

### Actual behavior

`CultureInfo.GetCultureInfo("root")` is accepted as a valid, recognized culture and does not throw a `CultureNotFoundException` - however what it produces is in fact **not** an actual intact culture.

### Regression?

_No response_

### Known Workarounds

_No response_

### Configuration

.NET version: .NET 10
OS: Windows 11

### Other information

_No response_

Start by reproducing the issue in a C# REPL with CultureInfo.GetCultureInfo("root").NumberFormat and inspect the CultureInfo and NumberFormat behavior described in the report. Trace the globalization implementation from CultureInfo.GetCultureInfo; done means "root" either throws CultureNotFoundException or returns an intact culture without the NullReferenceException.

Written by the indexing model from the issue text.