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/app-server/tests/common/models_cache.rs

@@ -0,0 +1,74 @@
+use chrono::DateTime;
+use chrono::Utc;
+use codex_core::openai_models::model_presets::all_model_presets;
+use codex_protocol::openai_models::ClientVersion;
+use codex_protocol::openai_models::ConfigShellToolType;
+use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::openai_models::ModelPreset;
+use codex_protocol::openai_models::ModelVisibility;
+use serde_json::json;
+use std::path::Path;
+
+/// Convert a ModelPreset to ModelInfo for cache storage.
+fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
+    ModelInfo {
+        slug: preset.id.clone(),
+        display_name: preset.display_name.clone(),
+        description: Some(preset.description.clone()),
+        default_reasoning_level: preset.default_reasoning_effort,
+        supported_reasoning_levels: preset.supported_reasoning_efforts.clone(),
+        shell_type: ConfigShellToolType::ShellCommand,
+        visibility: if preset.show_in_picker {
+            ModelVisibility::List
+        } else {
+            ModelVisibility::Hide
+        },
+        minimal_client_version: ClientVersion(0, 1, 0),
+        supported_in_api: true,
+        priority,
+        upgrade: preset.upgrade.as_ref().map(|u| u.id.clone()),
+        base_instructions: None,
+    }
+}
+
+/// Write a models_cache.json file to the codex home directory.
+/// This prevents ModelsManager from making network requests to refresh models.
+/// The cache will be treated as fresh (within TTL) and used instead of fetching from the network.
+/// Uses the built-in model presets from ModelsManager, converted to ModelInfo format.
+pub fn write_models_cache(codex_home: &Path) -> std::io::Result<()> {
+    // Get all presets and filter for show_in_picker (same as builtin_model_presets does)
+    let presets: Vec<&ModelPreset> = all_model_presets()
+        .iter()
+        .filter(|preset| preset.show_in_picker)
+        .collect();
+    // Convert presets to ModelInfo, assigning priorities (higher = earlier in list)
+    // Priority is used for sorting, so first model gets highest priority
+    let models: Vec<ModelInfo> = presets
+        .iter()
+        .enumerate()
+        .map(|(idx, preset)| {
+            // Higher priority = earlier in list, so reverse the index
+            let priority = (presets.len() - idx) as i32;
+            preset_to_info(preset, priority)
+        })
+        .collect();
+
+    write_models_cache_with_models(codex_home, models)
+}
+
+/// Write a models_cache.json file with specific models.
+/// Useful when tests need specific models to be available.
+pub fn write_models_cache_with_models(
+    codex_home: &Path,
+    models: Vec<ModelInfo>,
+) -> std::io::Result<()> {
+    let cache_path = codex_home.join("models_cache.json");
+    // DateTime<Utc> serializes to RFC3339 format by default with serde
+    let fetched_at: DateTime<Utc> = Utc::now();
+    let cache = json!({
+        "fetched_at": fetched_at,
+        "etag": null,
+        "models": models
+    });
+    std::fs::write(cache_path, serde_json::to_string_pretty(&cache)?)
+}