CmdPal: Fix missing primary context command for late-bound items (#46131)

This PR does fix a bug where an item that starts with a null or empty
primary command never adds that primary action to the context menu after
the extension later provides a real command.

- Creates the default primary context-menu item lazily when `Command` or
`Command.Name` becomes available after `SlowInitializeProperties()`
- Refreshes `AllCommands`, `SecondaryCommand`, and `HasMoreCommands`
notifications for late command materialization and Show Details updates.
- Adds unit tests to cover the fixed issue.


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## Summary of the Pull Request

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## PR Checklist

- [x] Closes: #46129 
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