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/Cargo.toml

@@ -79,7 +79,7 @@ tokio-stream = "0.1.17"
 tracing = { version = "0.1.41", features = ["log"] }
 tracing-appender = "0.2.3"
 tracing-subscriber = { version = "0.3.19", features = ["env-filter"] }
-tui-markdown = "0.3.3"
+pulldown-cmark = "0.10"
 unicode-segmentation = "1.12.0"
 unicode-width = "0.1"
 url = "2"