fix(github): Implement dynamic fallback for Codex models requiring /responses endpoint (#127)

* fix(github): add dynamic fallback to /responses for Codex models

* Refactor GithubExecutor: use config for URL detection

open-sse/config/constants.js

@@ -134,6 +134,7 @@ export const PROVIDERS = {
   },
   github: {
     baseUrl: "https://api.githubcopilot.com/chat/completions", // GitHub Copilot API endpoint for chat
+    responsesUrl: "https://api.githubcopilot.com/responses",
     format: "openai", // GitHub Copilot uses OpenAI-compatible format
     headers: {
       "copilot-integration-id": "vscode-chat",