Add /v1/messages endpoint for direct Anthropic forwarding

Features:
- Add new /v1/messages endpoint for transparent Anthropic request/response forwarding
- Only supports anthropic type endpoints (rejects openai with 400 error)
- No request transformation - forwards original request body as-is
- No response transformation - streams and non-streaming responses forwarded directly

Now supports three endpoint patterns:
- /v1/chat/completions: Universal with format conversion (anthropic, openai)
- /v1/responses: Direct proxy for openai endpoints only
- /v1/messages: Direct proxy for anthropic endpoints only

server.js

@@ -30,7 +30,8 @@ app.get('/', (req, res) => {
     endpoints: [
       'GET /v1/models',
       'POST /v1/chat/completions',
-      'POST /v1/responses'
+      'POST /v1/responses',
+      'POST /v1/messages'
     ]
   });
 });
@@ -88,7 +89,8 @@ app.use((req, res, next) => {
     availableEndpoints: [
       'GET /v1/models',
       'POST /v1/chat/completions',
-      'POST /v1/responses'
+      'POST /v1/responses',
+      'POST /v1/messages'
     ]
   });
 });
@@ -121,6 +123,7 @@ app.use((err, req, res, next) => {
       logInfo('  GET  /v1/models');
       logInfo('  POST /v1/chat/completions');
       logInfo('  POST /v1/responses');
+      logInfo('  POST /v1/messages');
     })
     .on('error', (err) => {
       if (err.code === 'EADDRINUSE') {