Bug: TUI hangs during startup with no MCP progress messages

[ivanmurashko]

Symptoms

• TUI appears to hang during startup when MCP servers are configured
• No "Booting MCP server: {name}" progress messages displayed
• Startup delay equals cumulative MCP server initialization time (~10-20 seconds at my case)
• User sees blank screen with no feedback

Root Cause

The blocking call to notify_sandbox_state_change() in Session::new() (lines 651-660 in codex.rs)
waits for ALL MCP servers to fully initialize before returning. This prevents the TUI event loop
from starting, so McpStartupUpdateEvent messages are emitted but never consumed or displayed.

Impact

• Poor user experience - app appears frozen during startup
• No visibility into MCP initialization progress

Introduced In

Commit c6f68c9df (PR #7112) - "feat: declare server capability in shell-tool-mcp"

Fix

Move sandbox state notification into each MCP server's background init task, sending the
notification immediately after the server becomes Ready. This allows Session::new() to return
immediately while still ensuring servers receive sandbox state before handling tool calls.