feat: add tower apps cancel command to cancel running runs

[socksy]

Expose the existing cancel_run API endpoint through the CLI and MCP server. Adds tower apps cancel <app_name> <run_number> as a subcommand under apps (consistent with existing apps show, apps logs, etc.) and a corresponding tower_apps_cancel MCP tool.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/cancel-runs-on-cli

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