Refactor: Add IsDisposed property to IShellPage and check before updating

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-23K

Steps used to test these changes

Stability is a top priority for Files and all changes are required to go through testing before being merged into the repo. Please include a list of steps that you used to test this PR.

1. Observed the reported issue in Sentry: FILES-APP-23K.
2. Verified that the fix addresses the issue by preventing ContentPageContext.Update() from accessing disposed ShellPage properties after navigation.
3. Confirmed that the IsDisposed property is correctly set in BaseShellPage.
4. Ensured that the check for ShellPage.IsDisposed in ContentPageContext.Update prevents updates after the page has been disposed.

This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 505535

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[hishitetsu]

I'm not very confident, but I don't think this makes sense.