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/suite/resume_warning.rs

@@ -42,7 +42,7 @@ fn resume_history(
 async fn emits_warning_when_resumed_model_differs() {
     // Arrange a config with a current model and a prior rollout recorded under a different model.
     let home = TempDir::new().expect("tempdir");
-    let mut config = load_default_config_for_test(&home);
+    let mut config = load_default_config_for_test(&home).await;
     config.model = Some("current-model".to_string());
     // Ensure cwd is absolute (the helper sets it to the temp dir already).
     assert!(config.cwd.is_absolute());