Compile the Java README quickstart offline

[kriptoburak]

Goal

Keep the Java README quickstart compatible with the released SDK.

Scope

• Add a minimal compile-only quickstart fixture.
• Mirror the README's first client-construction workflow.
• Use the supported Java 8 bytecode target.
• Do not send a network request.
• Run the fixture through the existing Gradle test path.

Acceptance Criteria

• The fixture compiles across the supported CI matrix.
• The repository's ./scripts/test command passes.
• README snippets and the fixture use the same public API.
• No credentials or remote endpoints are required.
• Keep generated SDK files and public contracts unchanged.

Starting Points

• README
• Core tests
• Contribution guide

This task should fit within 1 working day. It supports newcomer onboarding and OpenSSF evidence.