TUI: Remove core protocol dependency [1/7]

[etraut-openai]

Why

This is part 1 of a 7-PR stack to remove direct codex_protocol::protocol usage from codex-tui while keeping each layer reviewable and shippable.

This first layer reduces the size of the later chatwidget diff by mechanically moving MCP startup bookkeeping out of the central widget file without changing the event shapes or behavior.

What changed

• Extracted MCP startup status handling into tui/src/chatwidget/mcp_startup.rs.
• Kept the existing core event types in place for this purely mechanical move.
• Updated the MCP startup tests to import the moved test-only event types directly.

Verification

• cargo test -p codex-tui chatwidget::tests::mcp_startup

[fcoury-oai]

Did a smoke test with one MCP server that is always broken both on the current stable version and in this branch. Also compared the output of /mcp after boot and they are identical.

Code looks good, approved.