fix(filer.sync): resolve manifest chunks against source filer (#9705)

* fix(filer.sync): resolve manifest chunks against source filer

`UpdateEntry` was passing `filer.LookupFn(fs)` — the sink filer client —
into `compareChunks`. But `oldEntry`/`newEntry` chunks come from the
source cluster, so manifest resolution must hit the source filer's
volume servers. With two clusters that have overlapping volume IDs
(common once they grow past a few hundred volumes), the sink lookup
returns its own volume's URLs and the fetch 404s on the source's
fileKey:

  compare chunks error: fail to read manifest 631,0babe...: 404 Not Found

The 404 aborts the diff, the manifest chunk never gets replicated, and
the target ends up with whatever flat chunks happened to land from
earlier partial syncs — visible as `SIZE_MISMATCH` in filer.sync.verify
on files large enough to use chunk manifests (~150 GB+ in practice).

Only the manifest path was wrong; flat-chunk reads in `fetchAndWrite`
already use `fs.filerSource.ReadPart`.

* trim comment

* test(filer.sync): regression test for source-filer manifest lookup

Two recording filer gRPC servers stand in for source and sink. Driving
UpdateEntry with a manifest chunk and observing which one receives
LookupVolume proves compareChunks routes source-side lookups through
fs.filerSource, not fs. Reverting the fix flips the call onto the sink
filer and fails the assertion.

* drop test

weed/replication/sink/filersink/filer_sink.go

@@ -265,8 +265,8 @@ func (fs *FilerSink) UpdateEntry(key string, oldEntry *filer_pb.Entry, newParent
 		// this usually happens when the messages are not ordered
 		glog.V(2).Infof("late updates %s", key)
 	} else {
-		// find out what changed
-		deletedChunks, newChunks, err := compareChunks(context.Background(), filer.LookupFn(fs), oldEntry, newEntry)
+		// source-side chunks resolve via source filer; sink volume IDs may collide.
+		deletedChunks, newChunks, err := compareChunks(context.Background(), filer.LookupFn(fs.filerSource), oldEntry, newEntry)
 		if err != nil {
 			return true, fmt.Errorf("replicate %s compare chunks error: %v", key, err)
 		}