feat(api): add model field and root endpoint

Add a model field to the gemini request mapping and implement a new
root endpoint that returns a plain text status message.

src/mapper.ts

@@ -118,6 +118,7 @@ export async function mapRequest(body: RequestBody) {
 
   return {
     geminiReq: {
+      model: body.model,
       contents,
       generationConfig,
       stream: body.stream,

src/server.ts

@@ -24,7 +24,7 @@ if (VERBOSE) {
   consola.info('Verbose logging enabled');
 }
 
-consola.info('Google CLI OpenAI proxy');
+consola.info('Google CLI OpenAI API');
 
 // ==================================================================
 // HTTP Server Helpers
@@ -101,6 +101,12 @@ ensureOAuthCredentials()
           return;
         }
 
+        if (pathname === '/') {
+          res.writeHead(200, { 'Content-Type': 'text/plain' });
+          res.end('Google CLI OpenAI API server is running......');
+          return;
+        }
+
         if (!isAuthorized(req, res)) {
           return;
         }