[router][protocols] Add Axum validate extractor and use it for /v1/chat/completions endpoint (#11588)

sgl-router/tests/test_openai_routing.rs

@@ -601,7 +601,6 @@ async fn test_unsupported_endpoints() {
         prompt: None,
         text: Some("Hello world".to_string()),
         input_ids: None,
-        parameters: None,
         sampling_params: None,
         stream: false,
         return_logprob: false,
@@ -642,7 +641,6 @@ async fn test_openai_router_chat_completion_with_mock() {
     // Create a minimal chat completion request
     let mut chat_request = create_minimal_chat_request();
     chat_request.messages = vec![ChatMessage::User {
-        role: "user".to_string(),
         content: UserMessageContent::Text("Hello, how are you?".to_string()),
         name: None,
     }];