Simplify legacy lightpush autosharding path to a direct getShard call

The single-content-topic path went through getShardsFromContentTopics,
iterated the resulting table under the assumption it was 1-entry, and
had a defensive fallthrough (`resolved = false` → bare `return`) for
the empty-table case. That case is unreachable: on success the table
always has exactly one entry, and the fallthrough returned a default-
initialized Result — not behavior worth preserving.

Replace with the same two-step pattern the modern lightpushPublish uses
(NsContentTopic.parse → sharding.getShard) so both handlers share a
shape and the (now-explicit) failure paths surface useful error strings.

Verified against tests/node/test_wakunode_legacy_lightpush.nim (9/9).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

logos_delivery/waku/node/waku_node/lightpush.nim

@@ -128,17 +128,11 @@ proc legacyLightpushPublish*(
     else:
       if node.wakuAutoSharding.isNone():
         return err("Pubsub topic must be specified when static sharding is enabled")
-      let topicMap =
-        ?node.wakuAutoSharding.get().getShardsFromContentTopics(message.contentTopic)
-      var resolved = false
-      for pubsub, _ in topicMap.pairs: # There's only one pair anyway
-        pubsubForPublish = $pubsub
-        resolved = true
-        break
-      if not resolved:
-        # Preserve pre-existing behavior: an empty topicMap fell off the end
-        # of the loop and returned the default-initialized Result.
-        return
+      let parsedTopic = NsContentTopic.parse(message.contentTopic).valueOr:
+        return err("Invalid content-topic: " & $error)
+      let shard = node.wakuAutoSharding.get().getShard(parsedTopic).valueOr:
+        return err("Autosharding error: " & error)
+      pubsubForPublish = $shard
 
     let firstResult = await internalPublish(node, pubsubForPublish, msgWithProof, peer)