[Bug]: Version.VERSION is hardcoded and reports a stale version (1.0.13-SNAPSHOT vs 2.0.3-SNAPSHOT)

**Describe the bug**
`io.agentscope.core.Version#VERSION` is a hardcoded string literal that has drifted
out of sync with the actual Maven project version. As a result, the `User-Agent`
header sent to all model providers reports the stale version
`agentscope-java/1.0.13-SNAPSHOT` instead of the current `2.0.3-SNAPSHOT`.

**To Reproduce**
```java
import io.agentscope.core.Version;

System.out.println(Version.VERSION); // prints 1.0.13-SNAPSHOT
System.out.println(Version.getUserAgent()); // agentscope-java/1.0.13-SNAPSHOT; ...
```
Compare with the version declared in `pom.xml` (`2.0.3-SNAPSHOT`).

**Expected behavior**
`Version.VERSION` should match the Maven project version, so `getUserAgent()` reports
the version of the artifact actually being built/released.

**Environment**
- AgentScope-Java Version: 2.0.3-SNAPSHOT
- Java Version: 21
- OS: Windows 11

**Additional context**
Root cause: the version is duplicated in three places that must be manually kept in
sync — `pom.xml` ``, `Version.java` `VERSION`, and
`VersionTest#testVersionConstant`. Proposed fix: make `${project.version}` the single
source of truth and inject it at build time via Maven resource filtering (add a
`version.properties`, read it lazily from `Version`, and cross-check it in `VersionTest`).