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/session/mcp.rs

@@ -451,7 +451,7 @@ impl Session {
             mcp_config.auth_keyring_backend_kind,
             &turn_context.approval_policy,
             turn_context.sub_id.clone(),
-            self.get_tx_event(),
+            Some(self.get_tx_event()),
             mcp_startup_cancellation_token,
             turn_context.permission_profile(),
             mcp_runtime_context.clone(),

codex-rs/core/src/session/session.rs

@@ -1218,7 +1218,7 @@ impl Session {
                 config.auth_keyring_backend_kind(),
                 &session_configuration.approval_policy,
                 INITIAL_SUBMIT_ID.to_owned(),
-                tx_event.clone(),
+                Some(tx_event.clone()),
                 mcp_startup_cancellation_token,
                 session_configuration.permission_profile(),
                 mcp_runtime_context.clone(),