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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[JasonYeMSFT]

This feels like a broken header for a new section which is supposed to begin with "## Add new ..."

[github-actions]

Hi @vukelich. Thank you for your interest in helping to improve the Azure SDK experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[github-actions]

Hi @vukelich. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing /reopen if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the no-recent-activity label; otherwise, this is likely to be closed again with the next cleanup pass.