syntax-highlight bash lines (#3142)

i'm not yet convinced i have the best heuristics for what to highlight,
but this feels like a useful step towards something a bit easier to
read, esp. when the model is producing large commands.

<img width="669" height="589" alt="Screenshot 2025-09-03 at 8 21 56 PM"
src="https://github.com/user-attachments/assets/b9cbcc43-80e8-4d41-93c8-daa74b84b331"
/>

also a fairly significant refactor of our line wrapping logic.

codex-rs/core/src/lib.rs

@@ -7,7 +7,7 @@
 
 mod apply_patch;
 pub mod auth;
-mod bash;
+pub mod bash;
 mod chat_completions;
 mod client;
 mod client_common;