fix(gradle-plugin): use Worker API classLoaderIsolation to prevent Jackson version conflicts (#18753)

Move code generation into a Gradle Worker API WorkAction with
classLoaderIsolation, preventing buildscript classpath conflicts
(e.g. Jackson version mismatches from Spring Boot or other plugins).

- Extract generation logic into GenerateWorkAction/GenerateWorkParameters
- Always use classLoaderIsolation in GenerateTask.doWork()
- Add openApiGeneratorClasspath configuration for custom generator jars
- Wire missing enumNameMappings and operationIdNameMappings from extension to task
- Add TestKit tests for WorkAction property wiring

Author: sergeyk

modules/openapi-generator-gradle-plugin/build.gradle

@@ -47,6 +47,7 @@ dependencies {
 tasks.named("test", Test).configure {
     useTestNG()
     testLogging.showStandardStreams = false
+    systemProperty("openApiGeneratorVersion", openApiGeneratorVersion)
 
     beforeTest { descriptor ->
         logger.lifecycle("Running test: " + descriptor)

modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/OpenApiGeneratorPlugin.kt

@@ -62,6 +62,12 @@ class OpenApiGeneratorPlugin : Plugin<Project> {
 
             generate.outputDir.set(project.layout.buildDirectory.dir("generate-resources/main").map { it.asFile.path })
 
+            val openApiGeneratorClasspath = configurations.create("openApiGeneratorClasspath") {
+                isCanBeConsumed = false
+                isCanBeResolved = true
+                description = "Extra jars for the OpenAPI Generator worker (custom generators)"
+            }
+
             tasks.apply {
                 register("openApiGenerators", GeneratorsTask::class.java).configure {
                     group = pluginGroup
@@ -93,6 +99,8 @@ class OpenApiGeneratorPlugin : Plugin<Project> {
                     description =
                         "Generate code via Open API Tools Generator for Open API 2.0 or 3.x specification documents."
 
+                    generatorClasspath.setFrom(openApiGeneratorClasspath)
+
                     verbose.set(generate.verbose)
                     validateSpec.set(generate.validateSpec)
                     generatorName.set(generate.generatorName)
@@ -126,6 +134,8 @@ class OpenApiGeneratorPlugin : Plugin<Project> {
                     nameMappings.set(generate.nameMappings)
                     modelNameMappings.set(generate.modelNameMappings)
                     parameterNameMappings.set(generate.parameterNameMappings)
+                    enumNameMappings.set(generate.enumNameMappings)
+                    operationIdNameMappings.set(generate.operationIdNameMappings)
                     openapiNormalizer.set(generate.openapiNormalizer)
                     invokerPackage.set(generate.invokerPackage)
                     groupId.set(generate.groupId)