fix: keep the checksum of a zero length multipart object

hash.Checksum.AddPart returned before seeding the accumulator when the
part was empty, so a multipart object with no content at all ended up
with no checksum instead of the checksum of zero bytes. Completing such
an upload failed with XAmzContentChecksumMismatch when the client
supplied the correct object checksum, and stored an empty checksum when
it did not.

Run the type check and the first checksum seeding before the zero size
early return. Appending zero bytes still leaves an existing accumulator
unchanged, so only the all empty case changes: a zero length part
followed by content already merged correctly, because prepending no bytes
does not alter a CRC.

AddPart has a single production caller, the multipart completion path, and
its part checksum type is derived from the upload's own checksum type, so
the type check now reached for zero sized parts cannot fire there.

Add a table test over CRC32, CRC32C and CRC64NVME covering every position
an empty part can take, and an API level zero length full object upload
that exercises the persisted AppendTo/ReadCheckSums round trip.

Co-authored-by: ChatGPT <noreply@openai.com>
Co-authored-by: Claude <noreply@anthropic.com>

internal/hash/crc.go

@@ -35,18 +35,22 @@ func (c *Checksum) AddPart(other Checksum, size int64) error {
 	if !other.Type.CanMerge() {
 		return fmt.Errorf("checksum type cannot be merged")
 	}
-	if size == 0 {
-		return nil
-	}
 	if !c.Type.Is(other.Type.Base()) {
 		return fmt.Errorf("checksum type does not match got %s and %s", c.Type.String(), other.Type.String())
 	}
 	// If never set, just add first checksum.
+	// This must happen before the zero size check below, otherwise an object
+	// with no content at all never seeds the accumulator and ends up with no
+	// checksum instead of the checksum of zero bytes.
 	if len(c.Raw) == 0 {
 		c.Raw = other.Raw
 		c.Encoded = other.Encoded
 		return nil
 	}
+	// Appending zero bytes leaves the checksum unchanged.
+	if size == 0 {
+		return nil
+	}
 	if !c.Valid() {
 		return fmt.Errorf("invalid base checksum")
 	}