AutorecoveringModel.IsClosed should return true if model is unusable

[onematchfox]

Port of #1174 for main branch.

Side note/FYI: Commit on #1174 can be cherry-picked/backported to 7.x.

Proposed Changes

As per the description of IsClosed on IModel.cs, this property should

return[s] true if the model is no longer in a state where it can be used

At present this returns false if the _delegate has been disposed of which, I would say indicates that the model "is no longer in a state where it can be used".

Fixes (copy-paste?) regression introduced as part of 54f7840 by aligning the behaviour of IsClosed with the underlying implementation that would have been called were the delegate not disposed of.

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
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Further Comments

If approved then the same change will need to be applied to main although the commit in this PR can't be simply cherry-picked across. Let me know if you'd like me to submit a separate PR for this or if one of the maintainers will handle this.