Align C# Copilot LSP convention with upstream dotnet skills config

[Copilot]

The C# Copilot LSP convention had drifted from the upstream dotnet/skills configuration, so the published server definition no longer matched the current C# language server setup.

• Summary

  • Sync the convention with the upstream csharp LSP server definition from https://github.com/dotnet/skills/blob/main/plugins/dotnet/lsp.json.
  • Keep the convention’s behavior aligned with the current dotnet plugin expectations for C# editing.

• What changed

  • Updated conventions/copilot-lsp-csharp/convention.yml to mirror the upstream server configuration, including the current argument ordering, cwd, and warmupTimeoutMs.
  • Updated the C# file-extension mappings to use the upstream Razor-aware behavior for .razor and .cshtml.
  • Refreshed conventions/copilot-lsp-csharp/README.md so the documented behavior matches the published configuration.

[bgrainger]

I don't know if cwd and warmupTimeoutMs are supported (with this YAML syntax) or not.