Adds a unit test to inspect whether we mutate any public properties in the DOM by:

• Adding a complete OpenApi document - with 3.1 features and loading it into a DOM
• Serializing the DOM using a standard serializer - System.Text.JSON
• Serializing the same DOM using our current serialization infrastructure
• Serializing the DOM again using STJ
• Doing a deep comparison for the two STJ-serialized versions. If they're different, our custom serialization likely mutated the public DOM.

Fixes #2080