Keep the composer editable during TUI startup (#38642)

## Why

Configuration and app-server initialization can take time before the main TUI is ready, leaving users unable to begin drafting a prompt.

## What changed

- Show a provisional composer while startup work runs and carry its text, cursor position, paste state, and attachments into the initialized chat.
- Limit the provisional composer to safe editing and cancellation, and quarantine input around session pickers, approvals, and other actionable startup screens.
- Preserve configuration validation before terminal checks and recover terminal state cleanly from startup failures and caught panics.

## Testing

- Cover startup editing, multiline and large pastes, keymaps, cancellation, session-picker handoff, approval boundaries, and draft restoration.
- Verify non-interactive launches report configuration errors before terminal errors.

GitOrigin-RevId: fe04a85cfbbcff21ff87fa81fd17474827858575

codex-rs/tui/src/chatwidget.rs

@@ -1738,7 +1738,6 @@ impl ChatWidget {
         self.bottom_pane.pending_thread_approvals()
     }
 
-    #[cfg(test)]
     pub(crate) fn has_active_view(&self) -> bool {
         self.bottom_pane.has_active_view()
     }