feat: introduce CellWidget trait (#1148)

The motivation behind this PR is to make it so a `HistoryCell` is more
like a `WidgetRef` that knows how to render itself into a `Rect` so that
it can be backed by something other than a `Vec<Line>`. Because a
`HistoryCell` is intended to appear in a scrollable list, we want to
ensure the stack of cells can be scrolled one `Line` at a time even if
the `HistoryCell` is not backed by a `Vec<Line>` itself.

To this end, we introduce the `CellWidget` trait whose key method is:

```
fn render_window(&self, first_visible_line: usize, area: Rect, buf: &mut Buffer);
```

The `first_visible_line` param is what differs from
`WidgetRef::render_ref()`, as a `CellWidget` needs to know the offset
into its "full view" at which it should start rendering.

The bookkeeping in `ConversationHistoryWidget` has been updated
accordingly to ensure each `CellWidget` in the history is rendered
appropriately.

codex-rs/tui/src/cell_widget.rs

@@ -0,0 +1,20 @@
+use ratatui::prelude::*;
+
+/// Trait implemented by every type that can live inside the conversation
+/// history list.  It provides two primitives that the parent scroll-view
+/// needs: how *tall* the widget is at a given width and how to render an
+/// arbitrary contiguous *window* of that widget.
+///
+/// The `first_visible_line` argument to [`render_window`] allows partial
+/// rendering when the top of the widget is scrolled off-screen.  The caller
+/// guarantees that `first_visible_line + area.height as usize` never exceeds
+/// the total height previously returned by [`height`].
+pub(crate) trait CellWidget {
+    /// Total height measured in wrapped terminal lines when drawn with the
+    /// given *content* width (no scrollbar column included).
+    fn height(&self, width: u16) -> usize;
+
+    /// Render a *window* that starts `first_visible_line` lines below the top
+    /// of the widget. The window’s size is given by `area`.
+    fn render_window(&self, first_visible_line: usize, area: Rect, buf: &mut Buffer);
+}