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/components/singlepass-cli-app.tsx

@@ -5,12 +5,7 @@ import type { FileOperation } from "../utils/singlepass/file_ops";
 
 import Spinner from "./vendor/ink-spinner"; // Third‑party / vendor components
 import TextInput from "./vendor/ink-text-input";
-import {
-  OPENAI_TIMEOUT_MS,
-  OPENAI_BASE_URL as _OPENAI_BASE_URL,
-  getBaseUrl,
-  getApiKey,
-} from "../utils/config";
+import { OPENAI_TIMEOUT_MS, getBaseUrl, getApiKey } from "../utils/config";
 import {
   generateDiffSummary,
   generateEditSummary,
@@ -399,8 +394,8 @@ export function SinglePassApp({
       });
 
       const openai = new OpenAI({
-        apiKey: getApiKey(config.provider ?? "openai"),
-        baseURL: getBaseUrl(config.provider ?? "openai"),
+        apiKey: getApiKey(config.provider),
+        baseURL: getBaseUrl(config.provider),
         timeout: OPENAI_TIMEOUT_MS,
       });
       const chatResp = await openai.beta.chat.completions.parse({