ensureContentScript fails on *.pixiebrix.com after an extension reload

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I mentioned this before but apparently I did not open an issue.

ensureContentScript doesn't take any action in some cases:

• when the extension was granted access to the host
  • because injection is handled by webext-dynamic-content-scripts
• when manifest already includes the host
  • because injection is handled by the browser

There's one small case that's not covered though:

• after an extension reload, the browser won't inject the extension onto existing tabs like webext-dynamic-content-scripts does
  • you can see this behavior when installing any new web extension and nothing happens until you refresh the open tabs

Marking this as low priority as extension reload/update is rare, but we actually do it on install, so it could potentially affect users during boarding.

Repro

0. Ensure you don't have *:/*/* (due to an injection bug)
1. Open pixiebrix.com
2. Reload extension
3. Click browser action on the pixiebrix.com tab

Real-world scenarios

• user has multiple pixiebrix.com tabs open, installs extension; PB won't work on those tabs
• the user is on app.pixiebrix.com and the extension is silently updated in the background

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Related

• Inform user of failed sidebar injection #4124
• Content script remains "ready" even after unloading #4190

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Fixed by #5899