fix(java_exception): populate dubbo generic exception message

[leno23]

Description

Related to apache/dubbo-go#3361 and apache/dubbo-go#3378.

This PR completes the hessian2-side part of moving Dubbo generic exception handling into dubbo-go-hessian2. DubboGenericException already carries ExceptionClass and ExceptionMessage; this change makes the constructor and Error() expose a stable detail message from those fields so callers do not need to duplicate message formatting in dubbo-go.

The behavior remains compatible with decoded Java exceptions: when DetailMessage is already populated by decoding, Error() keeps returning that decoded detail. When it is empty, Error() falls back to the generic exception class/message formatting that dubbo-go previously implemented locally.

Validation

• go test ./java_exception
• go test . -run TestException
• go test ./...
• git diff --check
• local audit script (OVERALL_EXIT=0)

[Copilot]

Pull request overview

This PR updates DubboGenericException to expose a stable error detail message derived from ExceptionClass/ExceptionMessage when DetailMessage is not already populated (e.g., by decoding), so downstream callers don’t need to duplicate formatting logic.

Changes:

• Populate DetailMessage in NewDubboGenericException using a shared message formatter.
• Update DubboGenericException.Error() to prefer decoded DetailMessage, with a fallback to formatted class/message.
• Add unit tests covering constructor behavior and Error() fallback/preference rules.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
java_exception/dubbo_generic_exception.go	Sets/derives a stable error message for DubboGenericException via constructor + Error() fallback formatting.
java_exception/dubbo_generic_exception_test.go	Adds tests verifying message formatting, fallback behavior, and decoded detail preference.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[CAICAIIs]

pls fix ci error

[Alanxtl]

@Snow-kal @xxs588 help review this and #396

[leno23]

Fixed the CI formatting failure by applying the imports-formatter result to java_exception/dubbo_generic_exception_test.go. Local validation passes: go test ./java_exception, go test ./..., and git diff --check.