pageserver: assert that keys belong to shard (#9943)

We've seen cases where stray keys end up on the wrong shard. This
shouldn't happen. Add debug assertions to prevent this. In release
builds, we should be lenient in order to handle changing key ownership
policies.

Touches #9914.

libs/utils/src/shard.rs

@@ -164,6 +164,12 @@ impl TenantShardId {
     }
 }
 
+impl std::fmt::Display for ShardNumber {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 impl std::fmt::Display for ShardSlug<'_> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(