openapi3: correctly cache and track synthetic discriminated union envelope shapes

[witemple-msft]

Closes #8677

• Adds a manuallyTrack method to the usage tracker, allowing the emitter to bind the usage of synthetic discriminated union envelopes to the union type's usage (this influences how the emitter decides to rename models, which it only does if a type has multiple usage).
• Caches the types of created envelopes by visibility if the union variant is transformed by the visibility.
  • If a variant is subject to visibility, but not transformed, the default envelope model is used so that all untransformed variants will share a schema.
  • This system is just to ensure that the correct number of envelope models are created, one for each active visibility transform that will result in a different schema.
• Adds a test against regressions that validates the envelopes are created with the correct structure, binding to the correct transformed models of the underlying variant types.

[github-actions]

All changed packages have been documented.

• ✅ @typespec/openapi3

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@typespec/openapi3 - fix ✏️

Addressed an issue where @discriminated union envelope schemas could sometimes have duplicate names in the context of visibility transforms.

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[witemple-msft]

Validation of bugfix: https://cadlplayground.z22.web.core.windows.net/prs/8711/?e=%40typespec%2Fopenapi3&c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwoKbW9kZWwgTSB7CiAgQHZpc2liaWxpdHkoTGlmZWN5Y2xlLkNyZWF0ZSkKICBwcm9wOiBzdHJpbmc7Cn0KCkBkaXNjcmltaW5hdGVkCnVuaW9uIFXFTHZhcmlhbnQ6IE0sxSpwdXQgb3AgY8VNT3JVcGRhdGUoQGJvZHkgxAU6IFUpOiBVOw%3D%3D&options=%7B%7D&vs=%7B%7D