s3api: copy the trailer checksum before reading the next trailer line (#11583)

* s3api: copy the trailer checksum before reading the next trailer line

parseChunkChecksum kept the checksum value as a sub-slice of the line
returned by bufio.Reader.ReadSlice, which is only valid until the next
read. When the trailer lines arrive in separate TCP segments, reading
x-amz-trailer-signature refills the buffer and overwrites the saved
value, so a correct upload fails with InvalidDigest ("The Content-Md5
you specified is not valid").

The AWS SDK for Java v2 (>= 2.30) on a Linux JDK sends the trailer that
way; about half of its signed streaming uploads failed.

Fixes #11582

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* s3api: reuse crc32 writer and trim comments in trailer split test

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/s3api/chunked_reader_v4.go

@@ -537,7 +537,7 @@ func parseChunkChecksum(b *bufio.Reader) (ChecksumAlgorithm, []byte, error) {
 					glog.V(3).Infof("multiple checksum headers found in trailer, using last: %s", key)
 				}
 				checksumAlgorithm = alg
-				checksum = value
+				checksum = bytes.Clone(value)
 			}
 			// Ignore other trailer headers like x-amz-trailer-signature
 		}