HeaderDictionary.Enumerator.Reset() does not work.

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

Calling `Reset()` on `HeaderDictionary.Enumerator` does not result in the enumerator being reset.

The issue is caused by the cast shown [here](https://github.com/dotnet/aspnetcore/blob/9f2b0880ff3c09db4a8915d62788a7d1ba62df64/src/Http/Http/src/HeaderDictionary.cs#L454) which results in the underlying enumerator being copied by value.

### Expected Behavior

The enumerator is reset to the initial state.

### Steps To Reproduce

https://github.com/CoryCharlton/HeaderDictionary_Enumerator_Reset

### Exceptions (if any)

_No response_

### .NET Version

9.0.203

### Anything else?

PR that implements proposed fix is [here](https://github.com/dotnet/aspnetcore/pull/61701).