Complete word appends `1` to parameter name

**Version Used**: 17.10.0 P6

**Steps to Reproduce**:
Request completion (or `Ctrl`+`Space`) at parameter name:

```cs
Task WorkAsync(CancellationToken cancellat$$ionToken)
{

}
```

https://github.com/dotnet/roslyn/assets/9047283/a84ed81e-0b67-412d-a05a-ce76bd22ddee

**Diagnostic Id**:
N/A

**Expected Behavior**:
Completions are displayed for suggested names e.g. `cancellation`, `token`, `cancellationToken`.

**Actual Behavior**:
Parameter name `cancellationToken` is completed to `cancellationToken1` without offering and displaying options.

Start by reproducing the completion request in the C# editor with the provided CancellationToken example and compare the expected suggestions with the actual insertion. Trace the parameter-name completion behavior in the Roslyn repository; done means the suggested names are displayed and choosing cancellationToken no longer appends 1.

Written by the indexing model from the issue text.