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/chat/terminal-header.tsx

@@ -9,6 +9,7 @@ export interface TerminalHeaderProps {
   version: string;
   PWD: string;
   model: string;
+  provider?: string;
   approvalPolicy: string;
   colorsByPolicy: Record<string, string | undefined>;
   agent?: AgentLoop;
@@ -21,6 +22,7 @@ const TerminalHeader: React.FC<TerminalHeaderProps> = ({
   version,
   PWD,
   model,
+  provider = "openai",
   approvalPolicy,
   colorsByPolicy,
   agent,
@@ -32,7 +34,7 @@ const TerminalHeader: React.FC<TerminalHeaderProps> = ({
       {terminalRows < 10 ? (
         // Compact header for small terminal windows
         <Text>
-          ● Codex v{version} – {PWD} – {model} –{" "}
+          ● Codex v{version} – {PWD} – {model} ({provider}) –{" "}
           <Text color={colorsByPolicy[approvalPolicy]}>{approvalPolicy}</Text>
           {flexModeEnabled ? " – flex-mode" : ""}
         </Text>
@@ -65,6 +67,10 @@ const TerminalHeader: React.FC<TerminalHeaderProps> = ({
             <Text dimColor>
               <Text color="blueBright">↳</Text> model: <Text bold>{model}</Text>
             </Text>
+            <Text dimColor>
+              <Text color="blueBright">↳</Text> provider:{" "}
+              <Text bold>{provider}</Text>
+            </Text>
             <Text dimColor>
               <Text color="blueBright">↳</Text> approval:{" "}
               <Text bold color={colorsByPolicy[approvalPolicy]} dimColor>