* s3api: ensure MD5 is calculated or reused during CopyObject
Fixes#8155
- Capture and reuse source MD5 for direct copies
- Calculate MD5 for small inline objects during copy
* s3api: refactor encryption logic and safe MD5 copying
- Extract duplicated bucket default encryption logic into helper
- Use safe append copy for MD5 slice to avoid shared modifications
* refactor
* avoids unnecessary MD5 recalculations for small files