test: add StringSlice Append coverage

[LY-19418]

Add test coverage for StringSlice SliceValue.Append.

This follows the existing SliceValue Replace coverage pattern
and only adds tests without changing runtime behavior.

Test:
go test ./...

[CLAassistant]

All committers have signed the CLA.

[tomasaschan]

Thank you! I don't see any issues with those change, but I'm curious: any particular reason for sending us this patch?

[LY-19418]

谢谢！我没看出这些改动有什么问题，但我很好奇：您发送这个补丁有什么特别的原因吗？

I was looking through the SliceValue implementations and their tests to better understand the codebase, and noticed that Append is implemented for stringSliceValue but didn't appear to have direct test coverage.
Since there is already coverage for Replace in TestSSAsSliceValue, I thought adding a small test for Append would help cover that existing behavior and serve as a regression test.
There isn't a bug report or issue behind it — I just noticed the missing coverage while exploring the codebase.

[LY-19418]

Thanks for reviewing and merging! @tomasaschan