[Bug]: OpenAPI3 does not output correct names for unions declared within a namespace

[jacobroschen]

Describe the bug

Union types do not follow the same naming rules as other types such as Model when declared within a namespace. This appears to be a regression that was introduced in 0.51.0, specifically the conversion of OpenAPI3 to the EmitterFramework.

0.50.x playground (correct)
0.51.x playground (incorrect)

This also contradicts the TypeSpec OpenAPI3 Emitter documentation, which states the following.

Inline defined models will result in an inline schema. Explicitly declared models will be defined in the components/schemas section with the TypeSpec name qualified by any enclosing namespaces.

Tsp

import "@typespec/openapi3";

using OpenAPI;

namespace Space1 {
  model Bar {
    bar?: string;
  }

  union Foo {
    foo: int32,
  }
}

@service
namespace Space2 {
  op create(p1?: Space1.Bar, p2?: Space1.Foo): {};
}

Actual Specification

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /:
    post:
      operationId: create
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                p1:
                  $ref: '#/components/schemas/Space1.Bar'
                p2:
                  $ref: '#/components/schemas/Foo'
components:
  schemas:
    Foo:
      type: integer
      format: int32
    Space1.Bar:
      type: object
      properties:
        bar:
          type: string

Note that the union Foo is not prefixed with its namespace Space1 when included in components.schemas. I expected it to be included as Space1.Foo, like Space1.Bar.

Expected Specification

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /:
    post:
      operationId: create
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                p1:
                  $ref: '#/components/schemas/Space1.Bar'
                p2:
                  $ref: '#/components/schemas/Space1.Foo'
components:
  schemas:
    Space1.Foo:
      type: integer
      format: int32
    Space1.Bar:
      type: object
      properties:
        bar:
          type: string

Reproduction

https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvb3BlbmFwaTMiOw0KDQp1c2luZyBPcGVuQVBJxRJuYW1lc3BhY2UgU8QGMSB7DQogIG1vZGVsIEJhcsYPICBiYXI%2FOiBzdHJpbmc7xBN9xD0gIHVuaW9uIEZvb8gpZm9vOiBpbnQzMizHJ8UqQHNlcnZpY2XRcTLGOW9wIGNyZWF0ZShwMT86xhsxLkJhciwgcDLKEUZvbyk6IHt9Ow0KfQ%3D%3D&e=%40typespec%2Fopenapi3&options=%7B%7D

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.