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/tools/handlers/shell.rs

@@ -358,9 +358,9 @@ mod tests {
         ));
     }
 
-    #[test]
-    fn shell_command_handler_to_exec_params_uses_session_shell_and_turn_context() {
-        let (session, turn_context) = make_session_and_context();
+    #[tokio::test]
+    async fn shell_command_handler_to_exec_params_uses_session_shell_and_turn_context() {
+        let (session, turn_context) = make_session_and_context().await;
 
         let command = "echo hello".to_string();
         let workdir = Some("subdir".to_string());