fix(heal): send the dry-run flag to remote bucket heal

remotePeerS3Client.HealBucket sent only the bucket name and the remove
flag, and HealBucketHandler built its heal options from those two
values. During a dry-run bucket heal every remote node therefore ran a
real one: it created the bucket directory on drives that lacked it, or
removed empty bucket directories when the bucket was missing from
every pool. Only the coordinating node honored DryRun.

Add a "dry-run" key to the peer message and read it into
HealOpts.DryRun on the receiving side, so healBucketLocal returns
before MakeVol and DeleteVol on every node. The message is a string
map: an older node ignores the key, and a missing key reads as false,
so a mixed-version cluster behaves as before until every node is
upgraded.

The regression calls the real client over a test grid connection to
the real handler. It fails on main, and it fails if either side of the
change is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/peer-rest-server.go

@@ -1291,8 +1291,10 @@ func (s *peerRESTServer) HealBucketHandler(mss *grid.MSS) (np *grid.JSON[madmin.
 	}
 
 	bucketDeleted := mss.Get(peerS3BucketDeleted) == "true"
+	dryRun := mss.Get(peerS3BucketDryRun) == "true"
 	res, err := healBucketLocal(context.Background(), bucket, madmin.HealOpts{
 		Remove: bucketDeleted,
+		DryRun: dryRun,
 	})
 	if err != nil {
 		return np, grid.NewRemoteErr(err)