Fix InvalidCastException in Can/ConvertTo methods from KeyConverter

[h3xds1nz]

Description

Sending this in now when #8215 was finally merged as I've discovered this during development of #9697 but wanted to wait until I can fix the TODOs in the unit tests as well.

Fixes InvalidCastException thrown from CanConvertTo when unboxing cast is performed on value that was not null but was of a different type than Key or the underlying storage (int), e.g. an object or a different value type.
Can* methods should not (and converters do not intentionally) throw in them but this is simple a programmer's error, it is not documented behavior either, hence the fix for this.

https://learn.microsoft.com/en-us/dotnet/api/system.windows.input.keyconverter.canconvertto?view=windowsdesktop-8.0

Fixes the same issue in ConvertTo by throwing the proper (NotSupported) exception.

https://learn.microsoft.com/en-us/dotnet/api/system.windows.input.keyconverter.convertto?view=windowsdesktop-8.0

Customer Impact

Less unexpected exceptions thrown.

Regression

No.

Testing

Local build, unit test.

Risk

There might be a concern that a code was passing int instead of Key which is the underlying storage type, so it survives the unboxing hard cast that was being performed but not the type is check. We may therefore for back-compat include check for both Key and int but other converters just check for their type, so in my honest opinion it would be best to unify the behaviour and mention in release notes.

Microsoft Reviewers: Open in CodeFlow