Advertise the Desktop app in graphical Linux sessions (#39656)

## What changed

- Add Linux-specific install and launch guidance to the generic and paid tooltip pools when `DISPLAY` or `WAYLAND_DISPLAY` is set.
- Keep the Linux tooltip hidden in WSL and headless sessions, and preserve platform-specific macOS and Windows behavior.

## Testing

- Add coverage for native graphical Linux, WSL, headless Linux, macOS, and Windows tooltip selection.

GitOrigin-RevId: cbe9435db064d5718a591db363dcb7cc1429d5ee

codex-rs/tui/tooltips.txt

@@ -11,7 +11,6 @@ Use /fork to branch the current chat into a new thread.
 Use /side to start a side conversation in a temporary fork without polluting the main thread.
 Use /init to create an AGENTS.md with project-specific guidance.
 Use /mcp to list configured MCP tools.
-Run `codex app` to open the Desktop app (it installs on macOS if needed).
 Use /personality to customize how Codex communicates.
 Use /rename to rename your threads for easier thread resuming.
 Use the OpenAI docs MCP for API questions; enable it with `codex mcp add openaiDeveloperDocs --url https://developers.openai.com/mcp`.