[BUG][JAVA] Invalid code generated: non-existent ApiClient is referred in toUrlQueryString

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

After upgrading from v7.6.0 to v7.7.0, we have noticed that the generated code is invalid.

• The generated model class has an import of a class ApiClient which is not generated.
• The generated model class has a function toUrlQueryString(String) that uses a static function valueToString from the class ApiClient which is not generated.
  The old code was

joiner.add(String.format("%sfoo%s=%s", prefix, suffix, URLEncoder.encode(String.valueOf(getFoo()), StandardCharsets.UTF_8).replaceAll("\\+", "%20")));

The new code is

joiner.add(String.format("%sfoo%s=%s", prefix, suffix, URLEncoder.encode(ApiClient.valueToString(getFoo()), StandardCharsets.UTF_8).replaceAll("\\+", "%20")));

openapi-generator version

7.7.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Example API v1
  description: A reference API
  contact: {}
  version: 2.0.0
paths:
  /demo:
    get:
      responses:
        '200':
          description: Test
          content:
            'application/json':
              schema:
                type: object
                properties:
                  foo:
                    type: string

Generation Details

language: java
library: native

				<groupId>org.openapitools</groupId>
				<artifactId>openapi-generator-maven-plugin</artifactId>
				<version>7.7.0</version>
				<configuration>
					<generatorName>java</generatorName>
					<skipValidateSpec>false</skipValidateSpec>
					<generateApis>false</generateApis>
					<generateSupportingFiles>false</generateSupportingFiles>
					<generateModelTests>false</generateModelTests>
					<generateModelDocumentation>false</generateModelDocumentation>
					<generateApiTests>false</generateApiTests>
					<generateApiDocumentation>false</generateApiDocumentation>
					<output>${project.build.directory}/generated-sources</output>
					<configOptions>
						<sourceFolder>openapi</sourceFolder>
						<useJakartaEe>true</useJakartaEe>
						<withXml>true</withXml>
						<serializationLibrary>jackson</serializationLibrary>
						<library>native</library>
						<dateLibrary>java8</dateLibrary>
						<disallowAdditionalPropertiesIfNotPresent>false</disallowAdditionalPropertiesIfNotPresent>
					</configOptions>
				</configuration>
				<executions>
					<execution>
						<id>demo</id>
						<goals>
							<goal>generate</goal>
						</goals>
						<phase>generate-sources</phase>
						<configuration>
							<inputSpec>src/main/openapi/demo.yaml</inputSpec>
							<configOptions>
								<modelPackage>de.demo</modelPackage>
							</configOptions>
						</configuration>
					</execution>
				</executions>

Steps to reproduce

1. save the file above as src/main/openapi/demo.yaml
2. add the above plugin configuration to a maven pom
3. run mvn generate-sources
4. check target/generated-sources/openapi/de/demo/DemoGet200Response.java

Related issues/PRs

seems to be a regression from #18743
https://github.com/OpenAPITools/openapi-generator/pull/18743/files#diff-2eefc7a98c8e6aff3fa4589d8f2c364e7214c35e98806786e8dae207c42352df

Workaround

Set supportUrlQuery to false when toUrlQueryString is not needed.

Reproduce the regression with the supplied demo.yaml and Maven configuration by running mvn generate-sources, then inspect the Java generator changes associated with PR #18743 and supportUrlQuery. Done means the generated DemoGet200Response.java no longer imports or calls the non-existent ApiClient when support files are disabled, while preserving the intended URL-query behavior.