s3: return IncompleteBody instead of 500 for truncated PUT bodies (#10186)

* s3: add IncompleteBody error code

* operation: tag a truncated source read as ErrTruncatedBody

The chunked uploader wraps both source-read failures and volume-upload failures,
and a mid-write volume server drop also carries io.ErrUnexpectedEOF. Tag only the
source read so callers can tell a truncated input apart from a server-side fault.

* s3: return IncompleteBody for a truncated PUT body

A client abort or reverse-proxy timeout truncates the request body mid-upload.
putToFiler mapped every streaming-upload failure to InternalError (500), which a
reverse proxy relays as a 502. Classify a source-read truncation as IncompleteBody
(400) so the response matches AWS and passes through. All S3 write paths share
putToFiler, so they all benefit.

weed/s3api/s3err/s3api_errors.go

@@ -155,6 +155,9 @@ const (
 	ErrKeyTooLongError
 
 	ErrNoSuchConfiguration
+
+	// Truncated request body (fewer bytes than Content-Length)
+	ErrIncompleteBody
 )
 
 // Error message constants for checksum validation
@@ -297,6 +300,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "We encountered an internal error, please try again.",
 		HTTPStatusCode: http.StatusInternalServerError,
 	},
+	ErrIncompleteBody: {
+		Code:           "IncompleteBody",
+		Description:    "You did not provide the number of bytes specified by the Content-Length HTTP header.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 
 	ErrInvalidPart: {
 		Code:           "InvalidPart",