feat(lang/omnisharp): switch neotest adapter to vstest (#6540)

## Description
Replace `neotest-dotnet` with `neotest-vstest` in the Omnisharp extras.

The maintainer of `neotest-dotnet` has announced possible archiving and
recommended moving toward a vstest-based approach
(see Issafalcon/neotest-dotnet#142).

In addition, `neotest-dotnet` often fails to reliably discover tests
(currently reproducible), while `neotest-vstest` works consistently in
those same scenarios.

`neotest-vstest` delegates to Microsoft’s VSTest engine, which provides:

- Framework-agnostic discovery and execution (xUnit, NUnit, MSTest,
etc.)
- Support for both C# and F#
- Parameterized test support
- Compatibility with Microsoft.Testing.Platform going forward

According to the `neotest-dotnet` maintainer, this vstest-based approach
is more reliable and maintainable than parser-based discovery and is the
recommended path going forward.
## Related Issue(s)
- Ref: Issafalcon/neotest-dotnet#142
<!--
  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
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## Screenshots

<!-- Add screenshots of the changes if applicable. -->

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

Co-authored-by: Christoph Schmidpeter <christoph.schmidpeter@gmail.com>

lua/lazyvim/plugins/extras/lang/omnisharp.lua

@@ -104,12 +104,12 @@ return {
     "nvim-neotest/neotest",
     optional = true,
     dependencies = {
-      "Issafalcon/neotest-dotnet",
+      "Nsidorenco/neotest-vstest",
     },
     opts = {
       adapters = {
-        ["neotest-dotnet"] = {
-          -- Here we can set options for neotest-dotnet
+        ["neotest-vstest"] = {
+          -- Here we can set options for neotest-vstest
         },
       },
     },