Multipart partNumber validation was inconsistent because some handlers still
enforced the hardcoded S3 default while others used the instance-level
mpMaxParts setting. This created a split API contract where changing mpMaxParts
did not reliably affect all relevant endpoints. This change routes all
partNumber upper-bound checks through a shared effective limit helper so
deployments that customize mpMaxParts get consistent behavior across GetObject,
HeadObject, UploadPart, and UploadPartCopy, while preserving the default
S3-compatible maximum when mpMaxParts is unset.
Ben McClelland
e146646665