fix: suppress ghost screen events when app is in background

[marandaneto]

💡 Motivation and Context

When a Flutter app goes to the background (AppLifecycleState.paused or inactive), the OS or certain plugins can trigger widget tree rebuilds. This causes the router to reconstruct routes, which fires PosthogObserver callbacks (didPush, didPop, didReplace), resulting in ghost $screen events even though the user isn't looking at the app.

This was reported in #352 with an MRE. Rather than requiring users to work around this with a propertiesExtractor, this fix handles it built-in within PosthogObserver.

Fixes the issue described in #352 (comment)

💚 How did you test it?

• Added 5 unit tests covering all observer methods (didPush, didPop, didReplace) with paused/inactive/resumed lifecycle states
• All 98 existing tests continue to pass

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR

[marandaneto]

@didyk i think this approach would be better?

[ioannisj]

Makes sense, just a couple of comments around implementation

[didyk]

@marandaneto thanks!