Inline suggestions not displayed in C# files, while Open Completions returns results

Does this issue occur when all extensions are disabled?: Yes/No

Does this issue occur when all extensions are disabled?: No / Not applicable, because reproducing the issue requires GitHub Copilot.

- VS Code Version: 1.136.1 (a44adf7f53e00964ab890f9f8758a334f1fc15bc)
- OS Version: Windows 10.0.26200 x64
- GitHub Copilot Chat: 0.64.1
- C# extension: 2.140.9
- C# Dev Kit: 3.20.199
- Visual Studio Tools for Unity: 1.3.1

Steps to Reproduce:

1. Open a C# file in a Unity project.
2. Type code and wait for a Copilot inline suggestion.
3. No inline suggestion is displayed.
4. Run `GitHub Copilot: Open Completions Panel`.
5. The panel successfully displays a generated completion.

Expected result:

Copilot suggestions should appear inline as ghost text in C# files.

Actual result:

Inline suggestions do not appear in C# files. They work in plain text files. The Open Completions Panel also works correctly for the same C# file.

Relevant settings:

```json
"editor.inlineSuggest.enabled": true,
"github.copilot.enable": {
"*": true,
"csharp": true,
"plaintext": true
}
```

Copilot output:
```
[fetchCompletions] Request .../completions finished with 200 status
[solutions] Open Copilot completion: [GizmoType.Circle, circlePresenter);]
```
Copilot diagnostics show successful HTTP 200 responses for all required endpoints. Reloading the VS Code window does not resolve the issue.