MM-62960: Support both webhub iteration scopes properly (#30224)

When Channel iteration mode is enabled, we need to ensure that
channel scoped events do not fall through to the connIndex.All()
condition. This is possible because there are multiple hubs
in a given system. So a single event will flow through all of them
and in some hubs, a channel scoped event might not have any connections.

In that case, we need to stop processing further.

https://mattermost.atlassian.net/browse/MM-62960

```release-note
NONE
```

server/channels/api4/post_test.go

@@ -3354,8 +3354,25 @@ func TestPermanentDeletePost(t *testing.T) {
 }
 
 func TestWebHubMembership(t *testing.T) {
-	th := Setup(t).InitBasic()
-	defer th.TearDown()
+	t.Run("WithChannelIteration", func(t *testing.T) {
+		th := SetupConfig(t, func(cfg *model.Config) {
+			*cfg.ServiceSettings.EnableWebHubChannelIteration = true
+		}).InitBasic()
+		defer th.TearDown()
+
+		_testWebHubMembership(th, t)
+	})
+
+	t.Run("WithoutChannelIteration", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+
+		_testWebHubMembership(th, t)
+	})
+}
+
+func _testWebHubMembership(th *TestHelper, t *testing.T) {
+	t.Helper()
 
 	u1 := th.CreateUser()
 	th.LinkUserToTeam(u1, th.BasicTeam)