[GoClient] Fixed StringIndexOutOfBoundsException when multiple properties has the same required $ref oneOf objects

Created by: kandaaaaa

Go client generator will hit StringIndexOutOfBoundsException when the object (petPreference) has multiple (2+) required fields (pet1, pet2, pet3) that pointing to the same object (pet).

The code triggering the error coming from the processedMap that checks if the same model is in an infinite loop, and returns an empty string when more than two stimes cause this error.

The fix introduces the depth of the recursion to the function and the processedMap, so the processedMap tracks the depth of the model wherever it processes

Example

    pet:
      oneOf:
        - $ref: '#/components/schemas/dog'
        - $ref: '#/components/schemas/cat'
      discriminator:
        propertyName: attr
        mapping:
          DOG: '#/components/schemas/dog'
          CAT: '#/components/schemas/cat'

    petPreference:
      type: object
      required:
        - pet1
        - pet2
        - pet3
      properties:
        pet1:
          $ref: '#/components/schemas/pet'
        pet2:
          $ref: '#/components/schemas/pet'
        pet3:
          $ref: '#/components/schemas/pet'

Stacktrace:

[main] INFO  o.o.codegen.TemplateManager - writing file /local/gen/docs/PetPreference.md                                                                                                                 
Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Pet'                                                                                                             
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:665)                                                                                                             
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:891)                                                                                                                 
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:441)                         
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)                                                                                                     
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)                                                                                                                      
Caused by: java.lang.StringIndexOutOfBoundsException: String index out of range: -1                                                                                                                      
        at java.base/java.lang.String.substring(Unknown Source)                                                                                                                                          
        at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:632)                                                                                             
        at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:593)                                                                                             
        at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:640)                                                                                             
        at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:523)                                                                                             
        at org.openapitools.codegen.languages.GoClientCodegen.postProcessOperationsWithModels(GoClientCodegen.java:460)                                                                                  
        at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1212)  
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:568)        
        ... 4 more  

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