Support threadless MCP connections without event channels (#34408)

## What changed

- Make the MCP connection manager's event sender optional.
- Skip startup notifications when a caller has no session event stream.
- Decline interactive MCP elicitations in that mode while allowing the
  underlying operation to continue.
- Update threadless resource reads, status snapshots, and connector discovery
  to use the channel-free path.

## Testing

- Extend the app-server MCP resource test to verify that a threadless resource
  read declines an elicitation and still returns the resource contents.

GitOrigin-RevId: 65fea152adbc02a78e09a40cae4134b6dccffd34

codex-rs/codex-mcp/src/elicitation.rs

@@ -163,7 +163,7 @@ impl ElicitationRequestManager {
     pub(crate) fn make_sender(
         &self,
         server_name: String,
-        tx_event: Sender<Event>,
+        tx_event: Option<Sender<Event>>,
     ) -> SendElicitation {
         let router = self.router.clone();
         let approval_policy = self.approval_policy.clone();
@@ -233,6 +233,14 @@ impl ElicitationRequestManager {
                     }
                 }
 
+                let Some(tx_event) = tx_event else {
+                    return Ok(ElicitationResponse {
+                        action: ElicitationAction::Decline,
+                        content: None,
+                        meta: None,
+                    });
+                };
+
                 let public_request_id = format!(
                     "codex-mcp-elicitation-{}",
                     NEXT_ELICITATION_REQUEST_ID.fetch_add(1, Ordering::Relaxed)