Take Throwable#toString() into account while rendering stack traces in Pattern Layout

[JongminChung]

Align renderThrowableMessage(...) to use Throwable.toString(), restoring the default printStackTrace() behavior documented for the %ex converter.

• This resolves both Throwable pattern conversion loses Exception details #4019 and Uniformise %ex pattern and exception JTL resolver #3623.
• I added test case that issue reporter gives. (TestCase)

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Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (the build instructions)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests are provided

[JongminChung]

Hi, @vy

Would love to get your eyes on this when you get a moment.
Let me know if any changes are needed. Thanks!

[vy]

Could you add a changelog entry and make sure ./mvnw verify -pl :log4j-core,:log4j-core-test passes, please?

[vy]

Could you replace the entire AI slop description with a short meaningful summary, please?

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	✅
build-ubuntu-latest	clean install	3.9.8	✅
build-windows-latest	clean install	3.9.8	✅

Generated by gradle/develocity-actions

[JongminChung]

Could you replace the entire AI slop description with a short meaningful summary, please?

As this is my first PR, I wanted to provide more background context. I've shortened it now.
Thanks for the thorough review!

[vy]

@JongminChung, thanks for the update.

Please, don't ever force-push. It completely nukes all the conversation history.

Would you also mind adding a changelog entry file, please?

[JongminChung]

@JongminChung, thanks for the update.

Please, don't ever force-push. It completely nukes all the conversation history.

Would you also mind adding a changelog entry file, please?

Sorry, I deleted them thinking they were unnecessary commitments through force-push, but I overlooked that the context of the previous conversation disappeared.

Regarding the changelog you requested, I will read the document and write it.

Thank you for reviewing it in detail every time.