Support serialization of object examples on unions

[RichardCPoint]

This PR fixes this issue:

• [OpenAPI3] @example decorator not supported for unions #4869

Currently, the @typespec/openapi3 emitter supports @examples for models, but outputs incorrect examples for unions.

For example (playground link):

@example(#{ a: 1 })
union U {
  number: {
    a: integer,
  },
  text: {
    s: string,
  },
}

should output with { a: 1 } as the example, but instead it is {}.

The issue manifests in the serializeValueAsJson function in packages/compiler/src/lib/examples.ts, and occurs because the resolveUnions function in the same file fails to detect the appropriate union variant.

With this PR, the example is emitted correctly. I've added a unit test to confirm this.

[RichardCPoint]

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[chrisradek]

@RichardCPoint Thank you opening this pull request!

This fix looks great! Do you mind running npx chronus add from the repo root to create a changelog entry for this change? I'm also happy to run and push to your branch to add the changelog.

[RichardCPoint]

Do you mind running npx chronus add from the repo root to create a changelog entry for this change?

Thanks for the review! I've run the command above and committed the result – please let me know if there's anything else I should do.