[BUG][Java][Spring] Bean validation annotates required nullable fields with @NotNull

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The java bean validation annotates properties that are required but where the field is potentially null with the @NotNull annotation. This seems to be only the case when the field is not marked as readOnly.

For example the following schema

foo:
 required:
   - bar
  properties:
   bar:
     type: [number, "null"]

allows that the user provides

{"bar": null}

but the generated bean validation will reject it.

openapi-generator version

Tried it with both 7.12 and master (7a3ea28).

OpenAPI declaration file content or url

openapi: 3.1.1
info:
  title: Stripped petstore
  version: 1.0
paths:
  /pet:
    put:
      operationId: updatePet
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Pet'
        required: true
      responses:
        '200':
          description: Successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    Pet:
      required:
        - name
      type: object
      properties:
        name:
          type: [string, "null"]
          examples: [doggie]

Note: this is a 3.1 openapi, but this can probably easily be translated to a previous version.

Generation Details

Stripped version of the jersey3 config, but probably works with any beanValidation enabled java.

generatorName: jaxrs-jersey
outputDir: samples/server/petstore/jaxrs/jersey3
library: jersey3
additionalProperties:
  artifactId: jaxrs-jersey3-petstore-server
  hideGenerationTimestamp: "true"
  serverPort: "8082"

Steps to reproduce

Generate code with the given spec and config, observe the following model is generated

public class Pet   {
  public static final String JSON_PROPERTY_NAME = "name";
  @JsonProperty(JSON_PROPERTY_NAME)
  private String name;

....

  /**
   * Get name
   * @return name
   **/
  @JsonProperty(value = "name")
  @Schema(required = true, description = "")
  @NotNull 
  public String getName() {
    return name;
  }
....
}

Note the @NotNull on getName(), eventhough that null is explicitly marked as a valid value for the name property. I would expect that this is absent if null is a valid value for the property.

Adding verbose output I see in the ModelInfo for "name"

{ 
  "isString" : true,
   ...
   "isReadOnly" : false,
   "isWriteOnly" : false,
    "isNullable" : true,
}

so internally it correctly picks up the null-ability of the field.

Related issues/PRs

This issue was already partially discussed in #14765, but there it was stated that it is a separate point warranting a separate issue. I could not find such issue.

Suggest a fix

It looks like the 'beanValidation.mustache' is the source, where it says:
{{#required}}{{^isReadOnly}}@NotNull {{/isReadOnly}}{{/required}}
changing it to
{{#required}}{{^isReadOnly}}{{^isNullable}}@NotNull {{/isNullable}}{{/isReadOnly}}{{/required}}
fixes it, but I can't oversee if this has side effects.

Start with the beanValidation.mustache template and the ModelInfo fields shown in the report, then reproduce the issue with the provided Pet schema and jaxrs-jersey3 configuration. Done means generated Java bean validation accepts required properties whose OpenAPI type explicitly permits null, while retaining validation for non-nullable required properties and the existing readOnly behavior.