[core] do not always cast to ArraySchema

[jmini]

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Description of the PR

Similar to PR #3765 but solved at core Level.

Casting an ObjectSchema to ArraySchema after a ModelUtils.isArraySchema(schema) produces java.lang.ClassCastException: io.swagger.v3.oas.models.media.ObjectSchema cannot be cast to io.swagger.v3.oas.models.media.ArraySchema error in some cases.

This is because of the implementation of ModelUtils.isArraySchema(Schema) (the second part of the method):

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

Lines 361 to 370 in 5a54aa5

	public static boolean isArraySchema(Schema schema) {
	if (schema instanceof ArraySchema) {
	return true;
	}
	// assume it's an array if maxItems, minItems is set
	if (schema != null && (schema.getMaxItems() != null || schema.getMinItems() != null)) {
	return true;
	}
	return false;
	}

In the case commented with "// assume it's an array if maxItems, minItems is set" we have an ObjectSchema.
When we try to read the items value, we need to check if we are allowed to cast or not.

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[wing328]

@jmini thanks for the PR.

What about removing the following instead?

     // assume it's an array if maxItems, minItems is set 
     if (schema != null && (schema.getMaxItems() != null || schema.getMinItems() != null)) { 
         return true; 
     } 

As ModelUtils.isArraySchema(Schema) should be the source of truth to tell if a schema is ArraySchema or not.

[jmini]

@wing328: this was also my thought.
Maybe this was included a long time ago because checking for instanceOf ArraySchema was not sufficient. Maybe Swagger-Parser did improve in this area.

---

I will change this PR to remove the second part of the ModelUtils.isArraySchema(Schema) method, but I will keep the getSchemaItems() in the default codegen (it was a lot of copy-paste).

[jmini]

@wing328 ModelUtils.isArraySchema(Schema) is modified as discussed.

[wing328]

Maybe this was included a long time ago because checking for instanceOf ArraySchema was not sufficient.

That's what I recalled

[wing328]

LGTM