X-Amz-Decoded-Content-Length describes decoded aws-chunked payloads and must
not control a plain request. Using it for a plain UploadPart lets a larger
decoded header drive backend preallocation; the posix multipart path does not
truncate the staged part, so completion publishes a zero-filled tail.
Select the decoded length only for aws-chunked readers. Plain PutObject and
UploadPart now use Content-Length for both short-body validation and backend
allocation.
Ben McClelland
20a5778aac