[BUG][SPRING] response examples: uncompilable output, unsupported example types

Support for api response examples has been added to Spring generator in #20933 - i.e. in OpenAPI Generator 7.13.0. As discussed in this PR, there are still some known limitations to the current implementation:

• Conversion of examples to strings can lead to uncompilable output due to improper escaping:

  openapi: 3.0.3
  info:
    title: No examples in annotation example API
    version: 1.0.0
  paths:
    /:
      get:
        summary: Create a dog
        responses:
          '400':
            description: Bad Request
            content:
              application/json:
                schema:
                  $ref: '#/components/schemas/Error'
                examples:
                  dog age negative:
                    $ref: '#/components/examples/DogAgeNegativeError'
  
  components:
    schemas:
      Error:
        type: object
        properties:
          message:
            type: string
    examples:
      DogAgeNegativeError:
        value:
          message: '"' # <--- encoding problem here
  

• Only the examples keyword is supported, but not the example (singular) keyword:

  openapi: 3.0.3
  info:
    title: No examples in annotation example API
    version: 1.0.0
  paths:
    /:
      get:
        summary: Create a dog
        responses:
          '400':
            description: Bad Request
            content:
              application/json:
                schema:
                  $ref: '#/components/schemas/Error'
                example: # <--- not recognized
                  message: my description
  components:
    schemas:
      Error:
        type: object
        properties:
          message:
            type: string
  

• specifying examples inside the component schemas isn't supported

  openapi: 3.0.3
  info:
    title: No examples in annotation example API
    version: 1.0.0
  paths:
    /:
      get:
        summary: Create a dog
        responses:
          '400':
            description: Bad Request
            content:
              application/json:
                schema:
                  $ref: '#/components/schemas/Error'
  
  components:
    schemas:
      Error:
        type: object
        properties:
          message:
            type: string
        example: # <--- not supported
          message: my example string
  

• specifying examples in this alternative way shown in the spec results in empty names and no descriptions

Start by reading PR #20933 and reproducing the listed response-example cases in the Spring generator. Check generated output for escaping, singular and schema examples, and alternative Example Object metadata; done means the examples are represented correctly and the generated output compiles.