fix(testing): 🐛 isolate desktop UI tests and harden CLI contracts

[PrzemyslawKlys]

Summary

• isolate desktop-mutating and external-app UI tests behind explicit gates and move more coverage onto owned harness windows
• add a repo-local DesktopManager.TestApp for MCP/UI scenarios instead of relying on Notepad/Edge
• expand non-UI CLI contract coverage for help, dispatch, parsing, output formatting, and adapter mapping
• harden several desktop automation paths around process/window targeting, screenshots, and control input handling

Testing

• dotnet test Sources/DesktopManager.sln --nologo -m:1
• dotnet test Sources/DesktopManager.sln --nologo -m:1 --filter "FullyQualifiedNameCliApplicationTests|FullyQualifiedNameHelpTextTests"

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