[router] add grpc router pd mode for chat and generate (#11140)

sgl-router/src/protocols/spec.rs

@@ -2,6 +2,11 @@ use serde::{Deserialize, Serialize};
 use serde_json::{to_value, Map, Number, Value};
 use std::collections::HashMap;
 
+// Default model value when not specified
+fn default_model() -> String {
+    "unknown".to_string()
+}
+
 // # Protocol Specifications
 //
 // This module contains all protocol definitions for OpenAI and SGLang APIs.
@@ -169,6 +174,7 @@ pub struct ChatCompletionRequest {
     pub messages: Vec<ChatMessage>,
 
     /// ID of the model to use
+    #[serde(default = "default_model")]
     pub model: String,
 
     /// Number between -2.0 and 2.0. Positive values penalize new tokens based on their existing frequency in the text so far