feat(telemetry): integrate OpenTelemetry observability stack with health metrics

- Add OpenTelemetry SDK, OTLP exporter, Prometheus integration
- Implement connection tracking with active/total/disconnection metrics
- Add health endpoint with uptime and connection counts
- Integrate tracing spans for socket events and engine messages
- Add metrics collection for event handling duration
- Update health endpoint to include live runtime state
- Add graceful telemetry shutdown in main function
- Implement engine session active metrics tracking
- Add namespace-specific attributes to connection metrics
- Introduce message edit history retrieval endpoint
- Add scheduled message CRUD operations and dispatcher
- Update Socket.IO event registration with observability
- Refactor component update to remove dead code allowance
- Add comprehensive environment variables documentation
- Implement detailed development guidelines in AGENTS.md

socket/namespace.rs

@@ -75,7 +75,10 @@ impl Namespace {
     }
 
     /// Remove a socket by its socket SID.
-    pub async fn remove_socket_by_sid(&self, socket_sid: &str) {
+    ///
+    /// Returns `true` if a socket was actually removed, `false` if the SID
+    /// was not found (already removed or never existed).
+    pub async fn remove_socket_by_sid(&self, socket_sid: &str) -> bool {
         if let Some((_, socket)) = self.sockets.remove(socket_sid) {
             self.engine_to_socket.remove(&socket.engine_sid);
             self.remove_socket_from_local_rooms(socket_sid);
@@ -86,14 +89,19 @@ impl Namespace {
             {
                 tracing::warn!("Adapter del_all error for socket {}: {}", socket_sid, e);
             }
+            true
+        } else {
+            false
         }
     }
 
     /// Remove a socket by its engine SID (for engine-level disconnections).
-    pub async fn remove_socket(&self, engine_sid: &str) {
+    /// Returns `true` if a socket was actually removed.
+    pub async fn remove_socket(&self, engine_sid: &str) -> bool {
         if let Some((_, socket_sid)) = self.engine_to_socket.remove(engine_sid) {
-            self.remove_socket_by_sid(&socket_sid).await;
+            return self.remove_socket_by_sid(&socket_sid).await;
         }
+        false
     }
 
     /// Look up a socket by its socket SID.