Re-add markdown streaming (#2029)

Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.

codex-rs/tui/src/render/line_utils.rs

@@ -0,0 +1,45 @@
+use ratatui::text::Line;
+use ratatui::text::Span;
+
+/// Clone a borrowed ratatui `Line` into an owned `'static` line.
+pub fn line_to_static(line: &Line<'_>) -> Line<'static> {
+    Line {
+        style: line.style,
+        alignment: line.alignment,
+        spans: line
+            .spans
+            .iter()
+            .map(|s| Span {
+                style: s.style,
+                content: std::borrow::Cow::Owned(s.content.to_string()),
+            })
+            .collect(),
+    }
+}
+
+/// Append owned copies of borrowed lines to `out`.
+pub fn push_owned_lines<'a>(src: &[Line<'a>], out: &mut Vec<Line<'static>>) {
+    for l in src {
+        out.push(line_to_static(l));
+    }
+}
+
+/// Consider a line blank if it has no spans or only spans whose contents are
+/// empty or consist solely of spaces (no tabs/newlines).
+pub fn is_blank_line_spaces_only(line: &Line<'_>) -> bool {
+    if line.spans.is_empty() {
+        return true;
+    }
+    line.spans
+        .iter()
+        .all(|s| s.content.is_empty() || s.content.chars().all(|c| c == ' '))
+}
+
+/// Consider a line blank if its spans are empty or all span contents are
+/// whitespace when trimmed.
+pub fn is_blank_line_trim(line: &Line<'_>) -> bool {
+    if line.spans.is_empty() {
+        return true;
+    }
+    line.spans.iter().all(|s| s.content.trim().is_empty())
+}