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/providers.ts

@@ -0,0 +1,45 @@
+export const providers: Record<
+  string,
+  { name: string; baseURL: string; envKey: string }
+> = {
+  openai: {
+    name: "OpenAI",
+    baseURL: "https://api.openai.com/v1",
+    envKey: "OPENAI_API_KEY",
+  },
+  openrouter: {
+    name: "OpenRouter",
+    baseURL: "https://openrouter.ai/api/v1",
+    envKey: "OPENROUTER_API_KEY",
+  },
+  gemini: {
+    name: "Gemini",
+    baseURL: "https://generativelanguage.googleapis.com/v1beta/openai",
+    envKey: "GEMINI_API_KEY",
+  },
+  ollama: {
+    name: "Ollama",
+    baseURL: "http://localhost:11434/v1",
+    envKey: "OLLAMA_API_KEY",
+  },
+  mistral: {
+    name: "Mistral",
+    baseURL: "https://api.mistral.ai/v1",
+    envKey: "MISTRAL_API_KEY",
+  },
+  deepseek: {
+    name: "DeepSeek",
+    baseURL: "https://api.deepseek.com",
+    envKey: "DEEPSEEK_API_KEY",
+  },
+  xai: {
+    name: "xAI",
+    baseURL: "https://api.x.ai/v1",
+    envKey: "XAI_API_KEY",
+  },
+  groq: {
+    name: "Groq",
+    baseURL: "https://api.groq.com/openai/v1",
+    envKey: "GROQ_API_KEY",
+  },
+};