replace tui_markdown with a custom markdown renderer (#3396)

Also, simplify the streaming behavior.

This fixes a number of display issues with streaming markdown, and paves
the way for better markdown features (e.g. customizable styles, syntax
highlighting, markdown-aware wrapping).

Not currently supported:
- footnotes
- tables
- reference-style links

codex-rs/tui/src/streaming/controller.rs

@@ -380,16 +380,12 @@ mod tests {
         let expected = vec![
             "Loose vs. tight list items:".to_string(),
             "".to_string(),
-            "1. ".to_string(),
-            "Tight item".to_string(),
-            "2. ".to_string(),
-            "Another tight item".to_string(),
-            "3. ".to_string(),
-            "Loose item with its own paragraph.".to_string(),
+            "1. Tight item".to_string(),
+            "2. Another tight item".to_string(),
+            "3. Loose item with its own paragraph.".to_string(),
             "".to_string(),
-            "This paragraph belongs to the same list item.".to_string(),
-            "4. ".to_string(),
-            "Second loose item with a nested list after a blank line.".to_string(),
+            "   This paragraph belongs to the same list item.".to_string(),
+            "4. Second loose item with a nested list after a blank line.".to_string(),
             "    - Nested bullet under a loose item".to_string(),
             "    - Another nested bullet".to_string(),
         ];