s3api: preserve remote object ETags across cache fills (#11704)

* s3api: preserve remote object ETags across cache fills

* s3api: align remote copy validators and reject stale ETags

* s3api: keep remote ETag through metadata touches, drop it after resizes

The remote-ETag check required Mtime to match RemoteMtime, so a utimens
on a cold remote object returned an empty ETag until a later cache fill
restored the remote value. It also trusted LastLocalSyncTsNs alone, so a
setattr resize that leaves chunks and the stamp untouched kept serving
the old remote ETag for different local bytes.

Require FileSize to match RemoteSize for both the synced and remote-only
paths and stop comparing modification times: content changes already
surface through cleared sync stamps, chunks, or inline content, while a
metadata-only touch leaves the remote bytes authoritative. Regression
tests cover touched cold and cached entries and resized cached entries.

---------

Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>

weed/s3api/s3api_object_handlers_copy.go

@@ -601,27 +601,7 @@ func copyEntryETag(entry *filer_pb.Entry) string {
 	if entry == nil {
 		return ""
 	}
-	if entry.Extended != nil {
-		if etag, ok := entry.Extended[s3_constants.ExtETagKey]; ok && len(etag) > 0 {
-			return string(etag)
-		}
-	}
-	attr := filer.Attr{}
-	if entry.Attributes != nil {
-		attr = filer.Attr{
-			FileSize: entry.Attributes.FileSize,
-			Mtime:    time.Unix(entry.Attributes.Mtime, 0),
-			Crtime:   time.Unix(entry.Attributes.Crtime, 0),
-			Mime:     entry.Attributes.Mime,
-			Md5:      entry.Attributes.Md5,
-		}
-	}
-	return filer.ETagEntry(&filer.Entry{
-		Attr:    attr,
-		Chunks:  entry.Chunks,
-		Content: entry.Content,
-		Remote:  entry.RemoteEntry,
-	})
+	return strings.Trim(objectETag(entry), `"`)
 }
 
 func copyEntryToTarget(dst, src *filer_pb.Entry) {