[WIP] Update System.CommandLine to 2.0.0-beta6.25358.103

[Copilot]

Update System.CommandLine to 2.0.0-beta6.25358.103

This PR updates the System.CommandLine dependency from 2.0.0-beta4.22272.1 to 2.0.0-beta6.25358.103 and adapts the codebase to handle the API breaking changes introduced in the newer version.

Plan

• Explore repository structure and understand current System.CommandLine usage
• Update System.CommandLine version in Directory.Packages.props
• Identify and fix compilation errors resulting from API changes
• Apply migration changes based on the System.CommandLine 2.0 migration guide
• Test the updated command-line interface functionality
• Ensure all existing tests pass
• Verify the nbgv tool works correctly with the updated dependency

Current State

The main System.CommandLine usage is in src/nbgv/Program.cs which implements the command-line interface for the nbgv tool. The current implementation uses:

• Command and Option creation patterns
• SetHandler methods for command execution
• CommandLineBuilder for parser configuration

Fixes #1236.

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