feat: Add errorUnions feature testing endpoint

[bflad]

Reference: https://linear.app/speakeasy/issue/GEN-1461/feature-support-errorunions-feature-in-php

[linear]

GEN-1461 Feature: Support errorUnions feature in PHP

[bflad]

Turns out the other /errors/{status_code} endpoint can already work for this -- so if you see this you're probably looking for the errors-union-of-errors and errors-union-of-errors-discriminated tests and this OAS

  /errors/500#errorUnion:
    servers:
      - url: http://localhost:35456
    post:
      operationId: errorUnionPost
      tags:
        - errors
      requestBody:
        required: true
        content:
          application/json:
            schema:
              oneOf:
                - type: object
                  x-speakeasy-name-override: errorType1RequestBody
                  properties:
                    error:
                      type: string
                  required:
                    - error
                - type: object
                  x-speakeasy-name-override: errorType2RequestBody
                  properties:
                    error:
                      type: object
                      x-speakeasy-name-override: errorType2Message
                      properties:
                        message:
                          type: string
                      required:
                        - message
      responses:
        "200":
          description: OK
        "500":
          $ref: "components.yaml#/components/responses/errorUnionResponse"
  /errors/400#errorUnionDiscriminated:
    servers:
      - url: http://localhost:35456
    post:
      operationId: errorUnionDiscriminatedPost
      tags:
        - errors
      requestBody:
        required: true
        content:
          application/json:
            schema:
              oneOf:
                - type: object
                  x-speakeasy-name-override: taggedError1RequestBody
                  properties:
                    tag:
                      type: string
                    error:
                      type: string
                  required:
                    - tag
                    - error
                - type: object
                  x-speakeasy-name-override: taggedError2RequestBody
                  properties:
                    tag:
                      type: string
                    error:
                      type: object
                      x-speakeasy-name-override: taggedError2Message
                      properties:
                        message:
                          type: string
                      required:
                        - message
                  required:
                    - tag
                    - error
      responses:
        "200":
          description: OK
        "4XX":
          $ref: "components.yaml#/components/responses/errorUnionDiscriminatedResponse"