feat: support multiple providers via Responses-Completion transformation (#247)

https://github.com/user-attachments/assets/9ecb51be-fa65-4e99-8512-abb898dda569

Implemented it as a transformation between Responses API and Completion
API so that it supports existing providers that implement the Completion
API and minimizes the changes needed to the codex repo.

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>
Co-authored-by: Fouad Matin <169186268+fouad-openai@users.noreply.github.com>
Co-authored-by: Fouad Matin <fouad@openai.com>

codex-cli/src/components/singlepass-cli-app.tsx

@@ -5,7 +5,12 @@ 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 } from "../utils/config";
+import {
+  OPENAI_TIMEOUT_MS,
+  OPENAI_BASE_URL as _OPENAI_BASE_URL,
+  getBaseUrl,
+  getApiKey,
+} from "../utils/config";
 import {
   generateDiffSummary,
   generateEditSummary,
@@ -394,8 +399,8 @@ export function SinglePassApp({
       });
 
       const openai = new OpenAI({
-        apiKey: config.apiKey ?? "",
-        baseURL: OPENAI_BASE_URL || undefined,
+        apiKey: getApiKey(config.provider),
+        baseURL: getBaseUrl(config.provider),
         timeout: OPENAI_TIMEOUT_MS,
       });
       const chatResp = await openai.beta.chat.completions.parse({