Improve notifications sample with actionable and cancellable workflows

[Maycon Campos (mayc7n)]

Summary

This PR improves the notifications sample by demonstrating complete
notification workflows instead of isolated messages.
Changes

• return notification promises from registered commands
• demonstrate modal details and confirmation handling
• use typed MessageItem actions
• connect the warning's Retry action to the progress command
• implement cooperative progress cancellation
• clear pending timers and dispose cancellation listeners
• report deterministic progress stages totaling 100%
• wait for all commands in the Do Not Disturb example
• add the Node.js types required by the sample
• document the purpose and behavior of each notification pattern

Validation

• npm run compile
• npm run lint
• git diff --check

[Maycon Campos (mayc7n)]

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