[BUG][Java] ServerConfiguration is not included in imports when generating from simple OpenAPI file

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

When upgrading OpenAPI, I noticed this PR introduced ServerConfiguration for the restclient library but the generated code does not compile because the ServerConfiguration and ServerVariable POJOs are not imported.

openapi-generator version

7.15.x and 7.16.x

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Metric ACL API
  description: Metric ACL API for managing access control lists.
  version: 3.3.0
paths:
  # paths
components:
  securitySchemes:
    # security scheme definitions
  parameters:
    # parameter definitions
  headers:
    # header definitions
  schemas:
    # schema definitions

Generation Details

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>${openapi-generator.version}</version>
                <executions>
                    <execution>
                        <id>generate-java-http-client</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <output>${openapi.generation.directory}</output>
                            <inputSpec>${project.basedir}/src/main/resources/api-characteristic.yaml</inputSpec>
                            <generatorName>java</generatorName>
                            <generateApiTests>false</generateApiTests>
                            <generateModelTests>false</generateModelTests>
                            <generateApiDocumentation>false</generateApiDocumentation>
                            <generateModelDocumentation>false</generateModelDocumentation>
                            <generateSupportingFiles>true</generateSupportingFiles>
                            <supportingFilesToGenerate>ApiClient.java,Authentication.java,HttpBasicAuth.java,HttpBearerAuth.java,ApiKeyAuth.java,JavaTimeFormatter.java,RFC3339DateFormat.java</supportingFilesToGenerate>
                            <configOptions>
                                <sourceFolder>.</sourceFolder>
                                <groupId>${project.groupId}</groupId>
                                <invokerPackage>${openapi.generation.external.admintool.base.package}</invokerPackage>
                                <apiPackage>${openapi.generation.external.admintool.base.package}.api</apiPackage>
                                <modelPackage>${openapi.generation.external.admintool.base.package}.model</modelPackage>
                                <generateClientAsBean>true</generateClientAsBean>
                                <dateLibrary>java8</dateLibrary>
                                <artifactId>${openapi.generation.artifact.id.package}</artifactId>
                                <artifactVersion>${project.version}</artifactVersion>
                                <configPackage>${openapi.generation.external.admintool.base.package}.configuration</configPackage>
                                <basePackage>${openapi.generation.external.admintool.base.package}</basePackage>
                                <library>restclient</library>
                            </configOptions>
                            <typeMappings>
                                <typeMapping>OffsetDateTime=LocalDateTime</typeMapping>
                            </typeMappings>
                            <importMappings>
                                <importMapping>java.time.OffsetDateTime=java.time.LocalDateTime</importMapping>
                            </importMappings>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

• Run mvn install using provided OpenAPI spec file and Maven plugin config

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/pull/21699

Suggest a fix

Import the ServerConfiguration and ServerVariable imports in the mustache file.

Start by running mvn install with the minimal OpenAPI spec and Maven plugin configuration described in the issue, using the Java restclient library. Locate the Java supporting-file mustache template that generates the imports, add the missing ServerConfiguration and ServerVariable imports, and verify that the generated code compiles.