Update new-command.md for more CancellationToken examples and instructions

[vukelich]

There have been PRs since the series of "Great Cancellation" pull requests where some types aren't being generated with using CancellationToken. This PR is an attempt at incremental fixes to new-command.md for better async code generation.

[Copilot]

Pull request overview

This PR enhances the documentation in new-command.md to provide comprehensive guidance on proper CancellationToken usage in async methods, addressing issues where developers were omitting CancellationToken parameters in newly generated code.

Key Changes:

• Added explicit CancellationToken cancellationToken parameter to all async method signatures in code examples throughout the documentation
• Expanded guidance on when to use = default for CancellationToken parameters (only when other parameters also have defaults)
• Added critical checklist items and "Do not"/"Always" guidelines emphasizing proper CancellationToken usage in service interfaces, implementations, command methods, and tests

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