fix(tui): genericize image paste tooltip copy

[middleDuckAi]

What?

• make the image-paste tooltip copy generic in both TUI surfaces
• add focused tests that make sure the tooltip pool no longer hardcodes Ctrl+V

Why?

• current main already renders the footer shortcut differently by environment, so the remaining mismatch is the static tooltip text
• the existing issue comment suggesting an Alt+V footer change no longer matches current main

How?

• update codex-rs/tui/tooltips.txt
• update codex-rs/tui_app_server/tooltips.txt
• add regression coverage in both tooltips.rs modules

Testing

• cargo test -p codex-tui tooltips -- --nocapture
• cargo test -p codex-tui-app-server tooltips -- --nocapture

Refs openai#11708.