feat: add skipUnusedModels option to validate command and Gradle task

[fabb]

Description

Closes #17679

Adds a way to suppress the Unused model: <name> recommendation warnings emitted during spec validation for schemas that are defined in components/schemas but not directly referenced by any operation. This is a common pattern in specs that serve as shared model libraries or that are bundled across multiple spec files.

Prior art / existing workaround

RuleConfiguration already reads the openapi.generator.rule.unused-schemas system property to default enableUnusedSchemasRecommendation. This means the warning can already be suppressed without this PR, but only via undocumented JVM flags:

Gradle — add to gradle.properties:

org.gradle.jvmargs=-Dopenapi.generator.rule.unused-schemas=false

CLI — pass the flag when invoking the JVM:

JAVA_OPTS="-Dopenapi.generator.rule.unused-schemas=false" openapi-generator-cli validate -i api.yaml --recommend
# or
java -Dopenapi.generator.rule.unused-schemas=false -jar openapi-generator-cli.jar validate -i api.yaml --recommend

This PR surfaces the same control as a first-class, documented, per-invocation option.

Changes

CLI (validate command):

• New --skip-unused-models flag. When set, enableUnusedSchemasRecommendation is disabled on the RuleConfiguration before validation runs.

Gradle plugin (ValidateTask + OpenApiGeneratorValidateExtension):

• New skipUnusedModels property (default: false) on both the extension (for openApiValidate {} block) and the task.
• Wired through OpenApiGeneratorPlugin alongside the existing recommend and treatWarningsAsErrors properties.

Usage

CLI:

openapi-generator-cli validate -i api.yaml --recommend --skip-unused-models

Gradle (Groovy DSL):

openApiValidate {
  inputSpec = "$rootDir/api.yaml"
  skipUnusedModels = true
}

Gradle (Kotlin DSL):

openApiValidate {
  inputSpec.set("$rootDir/api.yaml")
  skipUnusedModels.set(true)
}

Tests

Added two new ValidateTaskDslTest cases and a companion test spec (petstore-v3.0-with-unused-schema.yaml):

• Verifies that the Unused model warning appears by default.
• Verifies that skipUnusedModels = true suppresses the warning while the build still succeeds.

[cubic-dev-ai]

No issues found across 6 files