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/Cargo.lock

@@ -1323,7 +1323,6 @@ dependencies = [
  "thiserror 2.0.17",
  "time",
  "tokio",
- "tokio-test",
  "tokio-util",
  "toml 0.9.5",
  "toml_edit",