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/protocol/src/protocol.rs

@@ -478,6 +478,12 @@ pub enum EventMsg {
     /// Ack the client's configure message.
     SessionConfigured(SessionConfiguredEvent),
 
+    /// Incremental MCP startup progress updates.
+    McpStartupUpdate(McpStartupUpdateEvent),
+
+    /// Aggregate MCP startup completion summary.
+    McpStartupComplete(McpStartupCompleteEvent),
+
     McpToolCallBegin(McpToolCallBeginEvent),
 
     McpToolCallEnd(McpToolCallEndEvent),
@@ -1383,6 +1389,37 @@ pub struct McpListToolsResponseEvent {
     pub auth_statuses: std::collections::HashMap<String, McpAuthStatus>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+pub struct McpStartupUpdateEvent {
+    /// Server name being started.
+    pub server: String,
+    /// Current startup status.
+    pub status: McpStartupStatus,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+#[serde(rename_all = "snake_case", tag = "state")]
+#[ts(rename_all = "snake_case", tag = "state")]
+pub enum McpStartupStatus {
+    Starting,
+    Ready,
+    Failed { error: String },
+    Cancelled,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS, Default)]
+pub struct McpStartupCompleteEvent {
+    pub ready: Vec<String>,
+    pub failed: Vec<McpStartupFailure>,
+    pub cancelled: Vec<String>,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+pub struct McpStartupFailure {
+    pub server: String,
+    pub error: String,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
 #[serde(rename_all = "snake_case")]
 #[ts(rename_all = "snake_case")]
@@ -1589,4 +1626,47 @@ mod tests {
         assert_eq!(deserialized, event);
         Ok(())
     }
+
+    #[test]
+    fn serialize_mcp_startup_update_event() -> Result<()> {
+        let event = Event {
+            id: "init".to_string(),
+            msg: EventMsg::McpStartupUpdate(McpStartupUpdateEvent {
+                server: "srv".to_string(),
+                status: McpStartupStatus::Failed {
+                    error: "boom".to_string(),
+                },
+            }),
+        };
+
+        let value = serde_json::to_value(&event)?;
+        assert_eq!(value["msg"]["type"], "mcp_startup_update");
+        assert_eq!(value["msg"]["server"], "srv");
+        assert_eq!(value["msg"]["status"]["state"], "failed");
+        assert_eq!(value["msg"]["status"]["error"], "boom");
+        Ok(())
+    }
+
+    #[test]
+    fn serialize_mcp_startup_complete_event() -> Result<()> {
+        let event = Event {
+            id: "init".to_string(),
+            msg: EventMsg::McpStartupComplete(McpStartupCompleteEvent {
+                ready: vec!["a".to_string()],
+                failed: vec![McpStartupFailure {
+                    server: "b".to_string(),
+                    error: "bad".to_string(),
+                }],
+                cancelled: vec!["c".to_string()],
+            }),
+        };
+
+        let value = serde_json::to_value(&event)?;
+        assert_eq!(value["msg"]["type"], "mcp_startup_complete");
+        assert_eq!(value["msg"]["ready"][0], "a");
+        assert_eq!(value["msg"]["failed"][0]["server"], "b");
+        assert_eq!(value["msg"]["failed"][0]["error"], "bad");
+        assert_eq!(value["msg"]["cancelled"][0], "c");
+        Ok(())
+    }
 }