Models: PropertyType constructor sets the DataTypeKey if IDataType has identity

[psot]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #20296

Description

• I added code to set the DataTypeKey on PropertyType constructor if the IDataType provided has identity, as it did for DataTypeId.
• If the DataTypeKey was not explicitly set after the PropertyType construction, this resulted in a constraint violation when saving the ContentType, and it could be easily overlooked.
• I also documented the DataTypeKey property on IPropertyType
• (closes DataTypeKey not set with PropertyType constructor when passing an IDataType #20296)

[github-actions]

Hi there @psot, thank you for this contribution! 👍

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[AndyButland]

Thanks very much for this @psot - all looks good to me. I just pushed some updates to the unit tests to reflect these changes, and will merge in once the build checks are complete.

[psot]

Thanks @AndyButland I did test it locally but didn't commit any of it, wasn't sure if you wanted and I'm not very familiar with any conventions used in tests so I skipped it. Next time. 😊

[AndyButland]

Don't worry - was all good. I just realised we already had some tests around this, so it made sense to assert this change too.