core/tui: non-blocking MCP startup (#6334)

This makes MCP startup not block TUI startup. Messages sent while MCPs
are booting will be queued.


https://github.com/user-attachments/assets/96e1d234-5d8f-4932-a935-a675d35c05e0


Fixes #6317

---------

Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/tui/src/history_cell.rs

@@ -1018,10 +1018,8 @@ fn try_new_completed_mcp_tool_call_with_image_output(
 }
 
 #[allow(clippy::disallowed_methods)]
-pub(crate) fn new_warning_event(message: String) -> PlainHistoryCell {
-    PlainHistoryCell {
-        lines: vec![vec![format!("⚠ {message}").yellow()].into()],
-    }
+pub(crate) fn new_warning_event(message: String) -> PrefixedWrappedHistoryCell {
+    PrefixedWrappedHistoryCell::new(message.yellow(), "⚠ ".yellow(), "  ")
 }
 
 #[derive(Debug)]