make model optional in config (#7769)

- Make Config.model optional and centralize default-selection logic in
ModelsManager, including a default_model helper (with
codex-auto-balanced when available) so sessions now carry an explicit
chosen model separate from the base config.
- Resolve `model` once in `core` and `tui` from config. Then store the
state of it on other structs.
- Move refreshing models to be before resolving the default model

codex-rs/common/src/config_summary.rs

@@ -4,10 +4,10 @@ use codex_core::config::Config;
 use crate::sandbox_summary::summarize_sandbox_policy;
 
 /// Build a list of key/value pairs summarizing the effective configuration.
-pub fn create_config_summary_entries(config: &Config) -> Vec<(&'static str, String)> {
+pub fn create_config_summary_entries(config: &Config, model: &str) -> Vec<(&'static str, String)> {
     let mut entries = vec![
         ("workdir", config.cwd.display().to_string()),
-        ("model", config.model.clone()),
+        ("model", model.to_string()),
         ("provider", config.model_provider_id.clone()),
         ("approval", config.approval_policy.to_string()),
         ("sandbox", summarize_sandbox_policy(&config.sandbox_policy)),