defer pruneCompletedChannelReqs until after onReadyToSend's index walk

Reviewer flagged that prune was mutating pendingMessagingRequests under
index iteration. Concrete failure: if prune drops the entry at the
current idx, the seq shifts and `idx.inc()` skips the next slot.

Hoist the single prune call to after the outer loop. Prune is idempotent
on the final-state filter, so doing it once at the end is equivalent to
calling it per-error-path while keeping idx aligned.

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

channels/reliable_channel.nim

@@ -220,7 +220,6 @@ proc onReadyToSend(
       ## parent `channelReqId` can still be pruned once its siblings
       ## are also final.
       self.pendingMessagingRequests[idx].segmentSendState = SegmentSendState.Failed
-      self.pruneCompletedChannelReqs()
       idx.inc()
       continue
     let wireBytes = seq[byte](encrypted)
@@ -254,7 +253,6 @@ proc onReadyToSend(
         ),
       )
       self.pendingMessagingRequests[idx].segmentSendState = SegmentSendState.Failed
-      self.pruneCompletedChannelReqs()
       idx.inc()
       continue
 
@@ -263,6 +261,10 @@ proc onReadyToSend(
     self.requestIds.mgetOrPut(channelReqId, @[]).add(messagingReqId)
     idx.inc()
 
+  ## Prune once after the index walk completes. Calling it inside the
+  ## loop would shift entries and misalign `idx` for the next iteration.
+  self.pruneCompletedChannelReqs()
+
 proc send*(
     self: ReliableChannel, payload: seq[byte], ephemeral: bool = false
 ): Result[RequestId, string] =