[Bug]: http-server-csharp: Some generated model files have a generic, numbered name

### Describe the bug

Supposedly, this model represents something like an operation result:

```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;

namespace Microsoft.MyRpNamespace;

public partial class Model10
{
///
/// The Location header contains the URL where the status of the long running operation can be checked.
///
[JsonPropertyName("location")]
public string Location { get; set; }

///
/// The Retry-After header can indicate how long the client should wait before polling the operation status.
///
[JsonPropertyName("retryAfter")]
public int? RetryAfter { get; set; }
}
```

But:

- File name: Model13.cs
- Class name: Model13

### 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 with the ARM RP reproduction and the @typespec/http-server-csharp emitter configured in tspconfig.yaml, then inspect the generated Model10.cs and Model13.cs files. Trace how the emitter assigns model names and verify that the operation-result model receives a meaningful name instead of a numbered one.

Written by the indexing model from the issue text.