fix(dcv): Optimize Ubuntu DCV installation and prevent restart prompts

[gmarciani]

• Add NEEDRESTART_MODE=l environment variable to suppress needrestart prompts during installation
• Install ubuntu-desktop with --no-install-recommends flag to reduce unnecessary package dependencies
• Update unit tests to verify NEEDRESTART_MODE is set and ubuntu-desktop installation uses --no-install-recommends
• Prevents interactive restart dialogs during automated DCV setup on Ubuntu systems

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