fix: inconsistent usage of base URL and API key (#507)

A recent commit introduced the ability to use third-party model
providers. (Really appreciate it!)

However, the usage is inconsistent: some pieces of code use the custom
providers, whereas others still have the old behavior. Additionally,
`OPENAI_BASE_URL` is now being disregarded when it shouldn't be.

This PR normalizes the usage to `getApiKey` and `getBaseUrl`, and
enables the use of `OPENAI_BASE_URL` if present.

---------

Co-authored-by: Gabriel Bianconi <GabrielBianconi@users.noreply.github.com>

codex-cli/src/utils/model-utils.ts

@@ -20,9 +20,11 @@ async function fetchModels(provider: string): Promise<Array<string>> {
     throw new Error("No API key configured for provider: " + provider);
   }
 
-  const baseURL = getBaseUrl(provider);
   try {
-    const openai = new OpenAI({ apiKey: getApiKey(provider), baseURL });
+    const openai = new OpenAI({
+      apiKey: getApiKey(provider),
+      baseURL: getBaseUrl(provider),
+    });
     const list = await openai.models.list();
     const models: Array<string> = [];
     for await (const model of list as AsyncIterable<{ id?: string }>) {