chore: cleanup Config instantiation codepaths (#8226)

This PR does various types of cleanup before I can proceed with more
ambitious changes to config loading.

First, I noticed duplicated code across these two methods:


https://github.com/openai/codex/blob/774bd9e432fa2e0f4e059e97648cf92216912e19/codex-rs/core/src/config/mod.rs#L314-L324


https://github.com/openai/codex/blob/774bd9e432fa2e0f4e059e97648cf92216912e19/codex-rs/core/src/config/mod.rs#L334-L344

This has now been consolidated in
`load_config_as_toml_with_cli_overrides()`.

Further, I noticed that `Config::load_with_cli_overrides()` took two
similar arguments:


https://github.com/openai/codex/blob/774bd9e432fa2e0f4e059e97648cf92216912e19/codex-rs/core/src/config/mod.rs#L308-L311

The difference between `cli_overrides` and `overrides` was not
immediately obvious to me. At first glance, it appears that one should
be able to be expressed in terms of the other, but it turns out that
some fields of `ConfigOverrides` (such as `cwd` and
`codex_linux_sandbox_exe`) are, by design, not configurable via a
`.toml` file or a command-line `--config` flag.

That said, I discovered that many callers of
`Config::load_with_cli_overrides()` were passing
`ConfigOverrides::default()` for `overrides`, so I created two separate
methods:

- `Config::load_with_cli_overrides(cli_overrides: Vec<(String,
TomlValue)>)`
- `Config::load_with_cli_overrides_and_harness_overrides(cli_overrides:
Vec<(String, TomlValue)>, harness_overrides: ConfigOverrides)`

The latter has a long name, as it is _not_ what should be used in the
common case, so the extra typing is designed to draw attention to this
fact. I tried to update the existing callsites to use the shorter name,
where possible.

Further, in the cases where `ConfigOverrides` is used, usually only a
limited subset of fields are actually set, so I updated the declarations
to leverage `..Default::default()` where possible.

codex-rs/app-server/src/codex_message_processor.rs

@@ -282,7 +282,7 @@ impl CodexMessageProcessor {
     }
 
     async fn load_latest_config(&self) -> Result<Config, JSONRPCErrorError> {
-        Config::load_with_cli_overrides(self.cli_overrides.clone(), ConfigOverrides::default())
+        Config::load_with_cli_overrides(self.cli_overrides.clone())
             .await
             .map_err(|err| JSONRPCErrorError {
                 code: INTERNAL_ERROR_CODE,
@@ -3348,7 +3348,7 @@ fn errors_to_info(
 
 async fn derive_config_from_params(
     overrides: ConfigOverrides,
-    cli_overrides: Option<std::collections::HashMap<String, serde_json::Value>>,
+    cli_overrides: Option<HashMap<String, serde_json::Value>>,
 ) -> std::io::Result<Config> {
     let cli_overrides = cli_overrides
         .unwrap_or_default()
@@ -3356,7 +3356,7 @@ async fn derive_config_from_params(
         .map(|(k, v)| (k, json_to_toml(v)))
         .collect();
 
-    Config::load_with_cli_overrides(cli_overrides, overrides).await
+    Config::load_with_cli_overrides_and_harness_overrides(cli_overrides, overrides).await
 }
 
 async fn read_summary_from_rollout(