volume server: drop the unused unmount result in VolumeCopy validation, and test-only EC helpers (#11531)

mount_and_reply ignored the Result of store.unmount_volume when a copied
replica failed record count validation, tripping unused_must_use. The Err
branch is unreachable there: the volume was mounted under the same store
write guard, mount_volume refuses an already loaded vid so it is a fresh
Volume with is_compacting false, and a compaction claim needs &mut Volume,
i.e. the store lock. Ignore the result explicitly with a one-line reason.

Store::delete_expired_ec_volumes and Store::remove_ec_volume are called
only from test modules (the heartbeat uses the split
find_expired_ec_volumes / remove_expired_ec_volumes halves), so mark them
#[cfg(test)].

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

seaweed-volume/src/server/grpc_server.rs

@@ -5683,7 +5683,8 @@ impl VolumeCopyJob {
                     v.deleted_count() as u64,
                 )
             {
-                store.unmount_volume(vid);
+                // Cannot fail: mounted under this same guard, so it cannot be compacting.
+                let _ = store.unmount_volume(vid);
                 return Err(Status::internal(format!(
                     "failed to mount or validate volume {}: {}",
                     vid, e

seaweed-volume/src/storage/store.rs

@@ -1216,6 +1216,7 @@ impl Store {
         found_vol.map(|v| (v, dirs))
     }
 
+    #[cfg(test)]
     pub fn delete_expired_ec_volumes(
         &mut self,
     ) -> (
@@ -1301,6 +1302,7 @@ impl Store {
     }
 
     /// Remove an EC volume from whichever location has it.
+    #[cfg(test)]
     pub fn remove_ec_volume(&mut self, vid: VolumeId) -> Option<EcVolume> {
         for loc in &mut self.locations {
             if let Some(ecv) = loc.remove_ec_volume(vid) {