AVRO-4322: [Java] Guard fast-reader-only assertions in FastReaderBuilderJavaClassTest - #3923

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: stCarolas/setup-maven@d6af6abeda15e98926a57b5aa970a96bb37f97d1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

[deprecation] setValidateDefaults(boolean) in Schema has been deprecated

[deprecation] setValidateDefaults(boolean) in Schema has been deprecated

[deprecation] getValidateDefaults() in Schema has been deprecated

[deprecation] toString(boolean) in Schema has been deprecated

[this-escape] possible 'this' escape before subclass is fully initialized

[deprecation] setNameValidator(NameValidator) in Schema has been deprecated

[deprecation] setNameValidator(NameValidator) in Schema has been deprecated

[deprecation] getNameValidator() in Schema has been deprecated

[this-escape] possible 'this' escape before subclass is fully initialized