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/utils/config.ts

@@ -10,6 +10,7 @@ import type { FullAutoErrorMode } from "./auto-approval-mode.js";
 
 import { log } from "./agent/log.js";
 import { AutoApprovalMode } from "./auto-approval-mode.js";
+import { providers } from "./providers.js";
 import { existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
 import { load as loadYaml, dump as dumpYaml } from "js-yaml";
 import { homedir } from "os";
@@ -40,12 +41,33 @@ export function setApiKey(apiKey: string): void {
   OPENAI_API_KEY = apiKey;
 }
 
+export function getBaseUrl(provider: string = "openai"): string | undefined {
+  const providerInfo = providers[provider.toLowerCase()];
+  if (providerInfo) {
+    return providerInfo.baseURL;
+  }
+  return undefined;
+}
+
+export function getApiKey(provider: string = "openai"): string | undefined {
+  const providerInfo = providers[provider.toLowerCase()];
+  if (providerInfo) {
+    if (providerInfo.name === "Ollama") {
+      return process.env[providerInfo.envKey] ?? "dummy";
+    }
+    return process.env[providerInfo.envKey];
+  }
+
+  return undefined;
+}
+
 // Formatting (quiet mode-only).
 export const PRETTY_PRINT = Boolean(process.env["PRETTY_PRINT"] || "");
 
 // Represents config as persisted in config.json.
 export type StoredConfig = {
   model?: string;
+  provider?: string;
   approvalMode?: AutoApprovalMode;
   fullAutoErrorMode?: FullAutoErrorMode;
   memory?: MemoryConfig;
@@ -76,6 +98,7 @@ export type MemoryConfig = {
 export type AppConfig = {
   apiKey?: string;
   model: string;
+  provider?: string;
   instructions: string;
   approvalMode?: AutoApprovalMode;
   fullAutoErrorMode?: FullAutoErrorMode;
@@ -270,6 +293,7 @@ export const loadConfig = (
       (options.isFullContext
         ? DEFAULT_FULL_CONTEXT_MODEL
         : DEFAULT_AGENTIC_MODEL),
+    provider: storedConfig.provider,
     instructions: combinedInstructions,
     notify: storedConfig.notify === true,
     approvalMode: storedConfig.approvalMode,
@@ -389,6 +413,7 @@ export const saveConfig = (
   // Create the config object to save
   const configToSave: StoredConfig = {
     model: config.model,
+    provider: config.provider,
     approvalMode: config.approvalMode,
   };