fix: when using --oss, ensure correct configuration is threaded through correctly (#1859)

This PR started as an investigation with the goal of eliminating the use
of `unsafe { std::env::set_var() }` in `ollama/src/client.rs`, as
setting environment variables in a multithreaded context is indeed
unsafe and these tests were observed to be flaky, as a result.

Though as I dug deeper into the issue, I discovered that the logic for
instantiating `OllamaClient` under test scenarios was not quite right.
In this PR, I aimed to:

- share more code between the two creation codepaths,
`try_from_oss_provider()` and `try_from_provider_with_base_url()`
- use the values from `Config` when setting up Ollama, as we have
various mechanisms for overriding config values, so we should be sure
that we are always using the ultimate `Config` for things such as the
`ModelProviderInfo` associated with the `oss` id

Once this was in place,
`OllamaClient::try_from_provider_with_base_url()` could be used in unit
tests for `OllamaClient` so it was possible to create a properly
configured client without having to set environment variables.

codex-rs/tui/src/lib.rs

@@ -10,6 +10,7 @@ use codex_core::config_types::SandboxMode;
 use codex_core::protocol::AskForApproval;
 use codex_core::util::is_inside_git_repo;
 use codex_login::load_auth;
+use codex_ollama::DEFAULT_OSS_MODEL;
 use log_layer::TuiLogLayer;
 use std::fs::OpenOptions;
 use std::io::Write;
@@ -71,25 +72,27 @@ pub async fn run_main(
         )
     };
 
+    // When using `--oss`, let the bootstrapper pick the model (defaulting to
+    // gpt-oss:20b) and ensure it is present locally. Also, force the built‑in
+    // `oss` model provider.
+    let model = if let Some(model) = &cli.model {
+        Some(model.clone())
+    } else if cli.oss {
+        Some(DEFAULT_OSS_MODEL.to_owned())
+    } else {
+        None // No model specified, will use the default.
+    };
+
     let model_provider_override = if cli.oss {
         Some(BUILT_IN_OSS_MODEL_PROVIDER_ID.to_owned())
     } else {
         None
     };
+
     let config = {
         // Load configuration and support CLI overrides.
         let overrides = ConfigOverrides {
-            // When using `--oss`, let the bootstrapper pick the model
-            // (defaulting to gpt-oss:20b) and ensure it is present locally.
-            model: if cli.oss {
-                Some(
-                    codex_ollama::ensure_oss_ready(cli.model.clone())
-                        .await
-                        .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?,
-                )
-            } else {
-                cli.model.clone()
-            },
+            model,
             approval_policy,
             sandbox_mode,
             cwd: cli.cwd.clone().map(|p| p.canonicalize().unwrap_or(p)),
@@ -154,6 +157,12 @@ pub async fn run_main(
         .with_target(false)
         .with_filter(env_filter());
 
+    if cli.oss {
+        codex_ollama::ensure_oss_ready(&config)
+            .await
+            .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?;
+    }
+
     // Channel that carries formatted log lines to the UI.
     let (log_tx, log_rx) = tokio::sync::mpsc::unbounded_channel::<String>();
     let tui_layer = TuiLogLayer::new(log_tx.clone(), 120).with_filter(env_filter());