Fix transfermanager multipart uploads for seekable bodies

[goutamadwant]

Fixes #3453

This updates UploadObject so multipart uploads for seekable bodies use section readers for each part instead of copying every part through the transfermanager buffer pool.

Seekable bodies are detected when the input implements both io.ReaderAt and io.Seeker. In that path, each multipart part is served with io.NewSectionReader, including when the body starts from a non-zero current offset. The existing buffered path is still used for non-seekable readers.

Validation:

• Added regression coverage for seekable multipart uploads using section readers and preserving part contents from a non-zero start offset.
• go test . -run TestUploadOrderMultiSeekableBodyUsesSectionReaders -count=1
• go test ./... -count=1

Changelog

No changelog entry added. Maintainers will handle the changelog entry.