chore: migrate from Config::load_from_base_config_with_overrides to ConfigBuilder (#8276)

https://github.com/openai/codex/pull/8235 introduced `ConfigBuilder` and
this PR updates all call non-test call sites to use it instead of
`Config::load_from_base_config_with_overrides()`.

This is important because `load_from_base_config_with_overrides()` uses
an empty `ConfigRequirements`, which is a reasonable default for testing
so the tests are not influenced by the settings on the host. This method
is now guarded by `#[cfg(test)]` so it cannot be used by business logic.

Because `ConfigBuilder::build()` is `async`, many of the test methods
had to be migrated to be `async`, as well. On the bright side, this made
it possible to eliminate a bunch of `block_on_future()` stuff.

codex-rs/core/tests/common/lib.rs

@@ -4,8 +4,8 @@ use tempfile::TempDir;
 
 use codex_core::CodexConversation;
 use codex_core::config::Config;
+use codex_core::config::ConfigBuilder;
 use codex_core::config::ConfigOverrides;
-use codex_core::config::ConfigToml;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use regex_lite::Regex;
 use std::path::PathBuf;
@@ -75,13 +75,13 @@ pub fn test_tmp_path_buf() -> PathBuf {
 /// Returns a default `Config` whose on-disk state is confined to the provided
 /// temporary directory. Using a per-test directory keeps tests hermetic and
 /// avoids clobbering a developer’s real `~/.codex`.
-pub fn load_default_config_for_test(codex_home: &TempDir) -> Config {
-    Config::load_from_base_config_with_overrides(
-        ConfigToml::default(),
-        default_test_overrides(),
-        codex_home.path().to_path_buf(),
-    )
-    .expect("defaults for test should always succeed")
+pub async fn load_default_config_for_test(codex_home: &TempDir) -> Config {
+    ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .harness_overrides(default_test_overrides())
+        .build()
+        .await
+        .expect("defaults for test should always succeed")
 }
 
 #[cfg(target_os = "linux")]

codex-rs/core/tests/common/test_codex.rs

@@ -178,7 +178,7 @@ impl TestCodexBuilder {
             ..built_in_model_providers()["openai"].clone()
         };
         let cwd = Arc::new(TempDir::new()?);
-        let mut config = load_default_config_for_test(home);
+        let mut config = load_default_config_for_test(home).await;
         config.cwd = cwd.path().to_path_buf();
         config.model_provider = model_provider;
         for hook in self.pre_build_hooks.drain(..) {