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/src/config/mod.rs

@@ -992,14 +992,13 @@ pub fn resolve_oss_provider(
 }
 
 impl Config {
-    /// Meant to be used exclusively for tests. For new tests, prefer using
-    /// [ConfigBuilder::build()], if possible, so ultimately we can make this
-    /// method private to this file.
-    pub fn load_from_base_config_with_overrides(
+    #[cfg(test)]
+    fn load_from_base_config_with_overrides(
         cfg: ConfigToml,
         overrides: ConfigOverrides,
         codex_home: PathBuf,
     ) -> std::io::Result<Self> {
+        // Note this ignores requirements.toml enforcement for tests.
         let requirements = ConfigRequirements::default();
         Self::load_config_with_requirements(cfg, overrides, codex_home, requirements)
     }