[kotlin] Add integration test for query params

[wing328]

based on #22588

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Summary by cubic

Add Kotlin Spring 3 RestClient integration tests to verify query parameter serialization for single values and lists, preventing regressions. Also updates the Gradle template and samples to include Spring Boot test support and minor build config fixes.

• New Features

  • Added PetApiTest and UserApiTest to assert correct query param encoding (e.g., status=pending,available and username/password).
  • Registered new test files in test_file_list.yaml.

• Dependencies

  • Added testImplementation "org.springframework.boot:spring-boot-test" to Kotlin Spring client projects (spring-2 webclient, spring-3 webclient/restclient).
  • Moved kotlintest test dependency to the common test block in the Kotlin client template.
  • Gated jvmToolchain(17) under useSpringBoot3 for the spring-webclient template and cleaned up sample build.gradle files.

^{Written for commit e8c422a. Summary will update on new commits.}

[cubic-dev-ai]

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