Allow Run to handle commandlines with spaces (#42016)

This better handles cases where commandlines might have embedded spaces.
For something like

```
C:\Program Files\PowerShell\7\pwsh.exe -c write-host dawg
```

we'll now see that `C:\Program` isn't a file, and we'll try to look at
`C:\Program Files\PowerShell\7\pwsh.exe` instead.

This code is pilfered from
https://github.com/microsoft/terminal/pull/12348 which fixed
https://github.com/microsoft/terminal/issues/12345. Terminal has great
code for normalizing a string into an executable and args, so why not
just use it here.

related to #41646
related to #41705 (but much more narrowly scoped)

----

I added some tests too.

drive-by fix: as I was adding tests, I added a helper for "make a change
to a page, and await the page's ItemsChanged". This removes a bunch of
`await 1s` calls, and brings the shell page tests from like, 7s to 500ms

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/FallbackExecuteItem.cs

@@ -92,7 +92,7 @@ internal sealed partial class FallbackExecuteItem : FallbackCommandItem, IDispos
             return;
         }
 
-        ShellHelpers.ParseExecutableAndArgs(searchText, out var exe, out var args);
+        ShellListPageHelpers.NormalizeCommandLineAndArgs(searchText, out var exe, out var args);
 
         // Check for cancellation before file system operations
         cancellationToken.ThrowIfCancellationRequested();