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-s3-server.go

@@ -29,6 +29,7 @@ import (
 const (
 	peerS3Bucket            = "bucket"
 	peerS3BucketDeleted     = "bucket-deleted"
+	peerS3BucketDryRun      = "dry-run"
 	peerS3BucketForceCreate = "force-create"
 	peerS3BucketForceDelete = "force-delete"
 )