Fix: Console launch fails when Windows username contains spaces - #12841

Summary

• Add null/empty guards in PathUtil.TryFindShellPath to prevent Path.Combine from throwing or producing invalid paths when ProgramW6432 env var is null or AppSettings.LinuxToolsDir is empty (both common when the Windows username contains spaces, e.g. "Acker Liu")
• Add unit tests for TryFindShellPath null-safety and Quote/QuoteNE with space-containing paths
• Add shell command quoting tests verifying GetChangeDirCommand and ExecutableCommandLine patterns produce correctly quoted output for paths with spaces

Fixes #11259

Test plan

• Run dotnet test --filter PathUtilTest — new tests pass, existing tests unaffected
• Run dotnet test --filter ShellTests — new shell quoting tests pass
• Manually verify console launch with a Windows user account containing spaces in the username