`dotnet run` can use the .slnx `DefaultStartup` property

### Is your feature request related to a problem?

Using `dotnet run` in a directory where there's only a solution or `dotnet run solution.slnx` currently shows an error:
> Couldn't find a project to run. Ensure a project exists in , or pass the path to the project using --project.

### Describe the solution you'd like

The .slnx format has the option to specify the startup project using ``. If there's a single one, `dotnet run` can use it to run the project.

### Alternatives you've considered

_No response_

### Additional context

_No response_

Start by tracing how `dotnet run` handles a directory containing only a solution and how it parses `.slnx` project entries. Verify the `DefaultStartup="true"` selection when exactly one project is marked, including `dotnet run solution.slnx`; done when that project runs while the existing error remains appropriate when no usable startup project is found.

Written by the indexing model from the issue text.