chore(upload): add offset/bytes-read context to chunk ReadFrom errors
Wrap io.ErrUnexpectedEOF (and siblings) from bytesBuffer.ReadFrom so
the log shows "read chunk at offset N (got M bytes): ..." instead of
a bare "unexpected EOF". The context distinguishes a client disconnect
before any data arrived (offset=0, got=0) from a mid-stream truncation
(offset>0, got<chunkSize) — diagnosing #9149 in the wild.
Verified at this stage that expectedDataSize is already threaded
correctly from upload_chunked.go (actual ReadFrom bytes) through
s3api assignFunc → filer AssignVolume → master PickForWrite.
No behavioral change to what the master receives.
Chris Lu
0a5c22b57e