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/core/src/lib.rs

@@ -32,6 +32,7 @@ pub use model_provider_info::BUILT_IN_OSS_MODEL_PROVIDER_ID;
 pub use model_provider_info::ModelProviderInfo;
 pub use model_provider_info::WireApi;
 pub use model_provider_info::built_in_model_providers;
+pub use model_provider_info::create_oss_provider_with_base_url;
 pub mod model_family;
 mod models;
 mod openai_model_info;

codex-rs/core/src/model_provider_info.rs

@@ -234,23 +234,6 @@ pub const BUILT_IN_OSS_MODEL_PROVIDER_ID: &str = "oss";
 pub fn built_in_model_providers() -> HashMap<String, ModelProviderInfo> {
     use ModelProviderInfo as P;
 
-    // These CODEX_OSS_ environment variables are experimental: we may
-    // switch to reading values from config.toml instead.
-    let codex_oss_base_url = match std::env::var("CODEX_OSS_BASE_URL")
-        .ok()
-        .filter(|v| !v.trim().is_empty())
-    {
-        Some(url) => url,
-        None => format!(
-            "http://localhost:{port}/v1",
-            port = std::env::var("CODEX_OSS_PORT")
-                .ok()
-                .filter(|v| !v.trim().is_empty())
-                .and_then(|v| v.parse::<u32>().ok())
-                .unwrap_or(DEFAULT_OLLAMA_PORT)
-        ),
-    };
-
     // We do not want to be in the business of adjucating which third-party
     // providers are bundled with Codex CLI, so we only include the OpenAI and
     // open source ("oss") providers by default. Users are encouraged to add to
@@ -295,29 +278,51 @@ pub fn built_in_model_providers() -> HashMap<String, ModelProviderInfo> {
                 requires_auth: true,
             },
         ),
-        (
-            BUILT_IN_OSS_MODEL_PROVIDER_ID,
-            P {
-                name: "Open Source".into(),
-                base_url: Some(codex_oss_base_url),
-                env_key: None,
-                env_key_instructions: None,
-                wire_api: WireApi::Chat,
-                query_params: None,
-                http_headers: None,
-                env_http_headers: None,
-                request_max_retries: None,
-                stream_max_retries: None,
-                stream_idle_timeout_ms: None,
-                requires_auth: false,
-            },
-        ),
+        (BUILT_IN_OSS_MODEL_PROVIDER_ID, create_oss_provider()),
     ]
     .into_iter()
     .map(|(k, v)| (k.to_string(), v))
     .collect()
 }
 
+pub fn create_oss_provider() -> ModelProviderInfo {
+    // These CODEX_OSS_ environment variables are experimental: we may
+    // switch to reading values from config.toml instead.
+    let codex_oss_base_url = match std::env::var("CODEX_OSS_BASE_URL")
+        .ok()
+        .filter(|v| !v.trim().is_empty())
+    {
+        Some(url) => url,
+        None => format!(
+            "http://localhost:{port}/v1",
+            port = std::env::var("CODEX_OSS_PORT")
+                .ok()
+                .filter(|v| !v.trim().is_empty())
+                .and_then(|v| v.parse::<u32>().ok())
+                .unwrap_or(DEFAULT_OLLAMA_PORT)
+        ),
+    };
+
+    create_oss_provider_with_base_url(&codex_oss_base_url)
+}
+
+pub fn create_oss_provider_with_base_url(base_url: &str) -> ModelProviderInfo {
+    ModelProviderInfo {
+        name: "gpt-oss".into(),
+        base_url: Some(base_url.into()),
+        env_key: None,
+        env_key_instructions: None,
+        wire_api: WireApi::Chat,
+        query_params: None,
+        http_headers: None,
+        env_http_headers: None,
+        request_max_retries: None,
+        stream_max_retries: None,
+        stream_idle_timeout_ms: None,
+        requires_auth: false,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     #![allow(clippy::unwrap_used)]