Migrate codex max (#7566)

- make codex max the default
- fix: we were doing some async work in sync function which caused tui
to panic

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__model_selection_popup.snap

@@ -5,9 +5,11 @@ expression: popup
   Select Model and Effort
   Access legacy models by running codex -m <model_name> or in your config.toml
 
-› 1. gpt-5.1-codex       Optimized for codex.
-  2. gpt-5.1-codex-mini  Optimized for codex. Cheaper, faster, but less
+› 1. gpt-5.1-codex-max   Latest Codex-optimized flagship for deep and fast
+                         reasoning.
+  2. gpt-5.1-codex       Optimized for codex.
+  3. gpt-5.1-codex-mini  Optimized for codex. Cheaper, faster, but less
                          capable.
-  3. gpt-5.1             Broad world knowledge with strong general reasoning.
+  4. gpt-5.1             Broad world knowledge with strong general reasoning.
 
   Press enter to select reasoning effort, or esc to dismiss.