[Bug]: http-server-csharp: Generated model file has unknown using TypeSpec.Helpers

### Describe the bug

Example:

```csharp
// Generated by @typespec/http-server-csharp
//
#nullable enable

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;
using TypeSpec.Helpers;
using TypeSpec.Helpers.JsonConverters;
// ^ The type or namespace name 'TypeSpec' could not be found (are you missing a using directive or an assembly reference?)

namespace Microsoft.MyRpNamespace;

public partial class Model5
{
}
```

### Reproduction

1. Use an ARM RP typespec
2. Add the following to tspconfig.yaml

```yml
emit:
- "@typespec/http-server-csharp"
options:
"@typespec/http-server-csharp":
emitter-output-dir: "{project-root}/generated/{emitter-name}"
output-type: all
emit-mocks: none
overwrite: true
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

Start by reproducing the issue with an ARM RP TypeSpec project using the shown tspconfig.yaml and the @typespec/http-server-csharp emitter. Inspect the generated model file and the emitter output for the TypeSpec.Helpers imports. Done means the generated C# model no longer references the unavailable TypeSpec.Helpers namespaces and compiles without the reported error.

Written by the indexing model from the issue text.