Polish shell integration message for slow shell startups

[Tyriar]

This message shows up in chat after waiting a default 5 seconds without shell integration showing up:

Enable shell integration to improve command detection

Let's explore:

• Showing it only once, when a terminal is created
• Showing it more detailed initially, pointing at the timeout setting or slow shell startup
• Revisiting _lastSuccessfulShell caching logic to make sure it's still relevant