feat: enhance custom_sandbox_example with full two-layer teaching demo

[flystar32]

Summary

Enhance the custom_sandbox_example to serve as a comprehensive teaching demo for both layers of sandbox customization.

Changes

New Files

• CustomClient.java — Fake/stub BaseClient implementation demonstrating the sandbox runtime instance lifecycle, with multi-platform comparison comments (Docker / K8s / ECS / Serverless)
• CustomClientStarter.java — Custom BaseClientStarter factory with Builder pattern
• README.md / README-zh.md — Bilingual documentation covering both infrastructure and application layer customization

Modified Files

• CustomSandbox.java — Added callTool() override with before/after/error hooks, fakeToolExecution(), and convenience methods (runPython() / runShell())
• Main.java — Rewritten as a 10-step end-to-end lifecycle demo using only custom components

Key Design Decisions

• All implementations use fake/stub code — no real runtime environment needed to run the example
• Uses "sandbox runtime backend" terminology instead of "container runtime backend" to reflect that the backend can be any compute resource (Docker, ECS, VM, K8s Pod, Serverless, etc.)
• API naming note added: although the code API uses "container" (e.g. createContainer, ContainerCreateResult), it is a general-purpose abstraction

How to Test

cd examples/simple_agent_use_examples/custom_sandbox_example
mvn clean compile
mvn exec:java -Dexec.mainClass="io.agentscope.Main"