refactor(apps): adopt WakuEnvelope in relay handlers and message-path bench

chat2 and networkmonitor relay handlers take the envelope; the micro/macro
benchmark's app handlers follow the new signature (paced/windowed publishing
and counters unchanged).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/benchmarks/message_path_bench.nim

@@ -211,7 +211,7 @@ proc emit(res: ScenarioResult) =
 # Node setup
 # ---------------------------------------------------------------------------
 
-proc dummyHandler(topic: PubsubTopic, msg: WakuMessage) {.async, gcsafe.} =
+proc dummyHandler(envelope: WakuEnvelope) {.async, gcsafe.} =
   discard
 
 proc buildIngestNode(
@@ -304,7 +304,7 @@ proc runMacro(shard: PubsubTopic, work: Workload): Future[ScenarioResult] {.asyn
     times: newSeqOfCap[MonoTime](work.msgs.len),
   )
 
-  proc countingHandler(topic: PubsubTopic, msg: WakuMessage) {.async, gcsafe.} =
+  proc countingHandler(envelope: WakuEnvelope) {.async, gcsafe.} =
     arrivals.times.add(getMonoTime())
     arrivals.count += 1
     if arrivals.count >= arrivals.target:

apps/chat2/chat2.nim

@@ -508,7 +508,9 @@ proc processInput(rfd: AsyncFD, rng: crypto.Rng) {.async.} =
 
   # Subscribe to a topic, if relay is mounted
   if conf.relay:
-    proc handler(topic: PubsubTopic, msg: WakuMessage): Future[void] {.async, gcsafe.} =
+    proc handler(envelope: WakuEnvelope): Future[void] {.async, gcsafe.} =
+      let topic = envelope.pubsubTopic
+      let msg = envelope.msg
       trace "Hit subscribe handler", topic
 
       if msg.contentTopic == chat.contentTopic:

apps/networkmonitor/networkmonitor.nim

@@ -518,9 +518,9 @@ proc subscribeAndHandleMessages(
     msgPerContentTopic: ContentTopicMessageTableRef,
 ) =
   # handle function
-  proc handler(
-      pubsubTopic: PubsubTopic, msg: WakuMessage
-  ): Future[void] {.async, gcsafe.} =
+  proc handler(envelope: WakuEnvelope): Future[void] {.async, gcsafe.} =
+    let pubsubTopic = envelope.pubsubTopic
+    let msg = envelope.msg
     trace "rx message", pubsubTopic = pubsubTopic, contentTopic = msg.contentTopic
 
     # If we reach a table limit size, remove c topics with the least messages.