Experimental JDK 21 compatibility check using JDK-Lite

Hi AgentScope Java maintainers,

We are currently working on a JDK compatibility experiment using our tool, **JDK-Lite**.

As part of this work, we forked this repository and performed an experimental compatibility check from JDK 17 toward JDK 21.

Experiment details:

- Upstream repository: https://github.com/agentscope-ai/agentscope-java
- Experimental fork: https://github.com/GG-Feng/agentscope-java
- Base commit tested: `7d3c6a11df22a8123459880e9bd5f6d0afed1108`
- Current JDK baseline considered in this experiment: JDK 17
- Target JDK version tested: JDK 21
- Status: experimental fixes prepared in our fork

We want to clarify that this is **not a request for AgentScope Java to upgrade its JDK baseline immediately**.

The purpose of this experiment is to collect and share compatibility evidence while keeping the current compatibility policy of the upstream project unchanged. Since this project targets the Java/JVM ecosystem, we hope the results may be useful if maintainers consider future JDK compatibility or baseline updates.

Summary of our current result:

- Compatibility target: JDK 21
- Fix status: we have prepared experimental fixes in our fork
- Main changes:
- Build/toolchain compatibility adjustments for the target JDK.
- Test/runtime compatibility adjustments discovered during the JDK-Lite experiment.
- Minimal changes required on our side to complete the experimental compatibility check.

The detailed changes can be reviewed in the experimental fork linked above.

If this information is useful, we would be happy to provide a more detailed compatibility report, split the changes into smaller patches, or prepare a PR in a form that matches the project’s contribution guidelines.

Thanks for maintaining AgentScope Java.