fix: make codex-mini-latest the default model in the Rust TUI (#972)

It's time to make `codex-mini-latest` the new default, as this should be
an "evergreen" model pointer.

* Equivalent change in TypeScript
https://github.com/openai/codex/pull/951
* See some notes about using `codex-mini-latest` with MCP in
https://github.com/openai/codex/pull/961

codex-rs/README.md

@@ -32,7 +32,7 @@ The `config.toml` file supports the following options:
 The model that Codex should use.
 
 ```toml
-model = "o3"  # overrides the default of "o4-mini"
+model = "o3"  # overrides the default of "codex-mini-latest"
 ```
 
 ### model_provider

codex-rs/core/src/flags.rs

@@ -3,7 +3,7 @@ use std::time::Duration;
 use env_flags::env_flags;
 
 env_flags! {
-    pub OPENAI_DEFAULT_MODEL: &str = "o4-mini";
+    pub OPENAI_DEFAULT_MODEL: &str = "codex-mini-latest";
     pub OPENAI_API_BASE: &str = "https://api.openai.com/v1";
 
     /// Fallback when the provider-specific key is not set.