Skip duplicate enum values

[czechboy0]

Motivation

Some OpenAPI docs, usually through imperfect conversion from another representation, end up with duplicate raw values in enum schemas.

The OpenAPI specification (by referencing the JSON Schema specification) says:

The value of this keyword MUST be an array.  This array SHOULD have at least one element.  Elements in the array SHOULD be unique.

So elements should be unique, but don't have to be. Today the generator fails to generate documents with such duplicate values.

Modifications

Gracefully handle duplicate values by emitting a warning diagnostic and skipping it.

Result

This unblocks generating OpenAPI documents with duplicate enum values, such as the Bluesky OpenAPI doc.

Test Plan

Added a unit test for duplicates.

[theoriginalbit]

I don't believe this applies to enums outside of the schema definitions, should it?

E.g. omit and raise a warning for duplicates for server enum definitions. I don't remember explicitly testing duplicates when I made the recent changes