fix: restore MCP startup progress messages in TUI (fixes #7827) (#7828)

## Problem

The introduction of `notify_sandbox_state_change()` in #7112 caused a
regression where the blocking call in `Session::new()` waits for all MCP
servers to fully initialize before returning. This prevents the TUI
event loop from starting, resulting in `McpStartupUpdateEvent` messages
being emitted but never consumed or displayed. As a result, the app
appears to hang during startup, and users do not see the expected
"Booting MCP server: {name}" status line.

Issue: [#7827](https://github.com/openai/codex/issues/7827)

## Solution
This change moves sandbox state notification into each MCP server's
background initialization task. The notification is sent immediately
after the server transitions to the Ready state. This approach:
- Avoids blocking `Session::new()`, allowing the TUI event loop to start
promptly.
- Ensures each MCP server receives its sandbox state before handling any
tool calls.
- Restores the display of "Booting MCP server" status lines during
startup.

## Key Changes
- Added `ManagedClient::notify_sandbox_state()` method.
- Passed sandbox_state to `McpConnectionManager::initialize()`.
- Sends sandbox state notification in the background task after the
server reaches Ready status.
- Removed blocking notify_sandbox_state_change() methods.
- Added a chatwidget snapshot test for the "Booting MCP server" status
line.

## Regression Details

Regression was bisected to #7112, which introduced the blocking
behavior.

---------

Co-authored-by: Michael Bolin <bolinfest@gmail.com>
Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/tui/src/chatwidget/tests.rs

@@ -27,6 +27,8 @@ use codex_core::protocol::ExecCommandSource;
 use codex_core::protocol::ExecPolicyAmendment;
 use codex_core::protocol::ExitedReviewModeEvent;
 use codex_core::protocol::FileChange;
+use codex_core::protocol::McpStartupStatus;
+use codex_core::protocol::McpStartupUpdateEvent;
 use codex_core::protocol::Op;
 use codex_core::protocol::PatchApplyBeginEvent;
 use codex_core::protocol::PatchApplyEndEvent;
@@ -2417,6 +2419,28 @@ fn status_widget_active_snapshot() {
     assert_snapshot!("status_widget_active", terminal.backend());
 }
 
+#[test]
+fn mcp_startup_header_booting_snapshot() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None);
+    chat.show_welcome_banner = false;
+
+    chat.handle_codex_event(Event {
+        id: "mcp-1".into(),
+        msg: EventMsg::McpStartupUpdate(McpStartupUpdateEvent {
+            server: "alpha".into(),
+            status: McpStartupStatus::Starting,
+        }),
+    });
+
+    let height = chat.desired_height(80);
+    let mut terminal = ratatui::Terminal::new(ratatui::backend::TestBackend::new(80, height))
+        .expect("create terminal");
+    terminal
+        .draw(|f| chat.render(f.area(), f.buffer_mut()))
+        .expect("draw chat widget");
+    assert_snapshot!("mcp_startup_header_booting", terminal.backend());
+}
+
 #[test]
 fn background_event_updates_status_header() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None);