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/core/src/connectors.rs

@@ -5,7 +5,6 @@ use std::sync::Mutex as StdMutex;
 use std::time::Duration;
 use std::time::Instant;
 
-use async_channel::unbounded;
 pub use codex_connectors::AppBranding;
 pub use codex_connectors::AppInfo;
 pub use codex_connectors::AppMetadata;
@@ -235,9 +234,6 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_mcp_manager(
     let runtime_context =
         McpRuntimeContext::new(Arc::clone(&environment_manager), config.cwd.to_path_buf());
 
-    let (tx_event, rx_event) = unbounded();
-    drop(rx_event);
-
     let cancel_token = CancellationToken::new();
     let codex_apps_auth_manager =
         codex_mcp::host_owned_codex_apps_enabled(&mcp_config, auth.as_ref())
@@ -248,7 +244,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_mcp_manager(
         config.auth_keyring_backend_kind(),
         &config.permissions.approval_policy,
         INITIAL_SUBMIT_ID.to_owned(),
-        tx_event,
+        /*tx_event*/ None,
         cancel_token.clone(),
         PermissionProfile::default(),
         // Connector discovery is threadless. Use an actually configured env if