[Router] Refactor protocol definitions: split spec.rs into modular files (#11677)

Co-authored-by: Chang Su <chang.s.su@oracle.com>

sgl-router/tests/spec/rerank.rs

@@ -1,9 +1,10 @@
 use serde_json::{from_str, to_string, Number, Value};
-use sglang_router_rs::protocols::spec::{
-    GenerationRequest, RerankRequest, RerankResponse, RerankResult, StringOrArray, UsageInfo,
-    V1RerankReqInput,
+use sglang_router_rs::protocols::common::{GenerationRequest, StringOrArray, UsageInfo};
+use sglang_router_rs::protocols::rerank::{
+    RerankRequest, RerankResponse, RerankResult, V1RerankReqInput,
 };
 use std::collections::HashMap;
+use validator::Validate;
 
 #[test]
 fn test_rerank_request_serialization() {
@@ -75,8 +76,7 @@ fn test_rerank_request_validation_empty_query() {
     };
 
     let result = request.validate();
-    assert!(result.is_err());
-    assert_eq!(result.unwrap_err(), "Query cannot be empty");
+    assert!(result.is_err(), "Should reject empty query");
 }
 
 #[test]
@@ -92,8 +92,7 @@ fn test_rerank_request_validation_whitespace_query() {
     };
 
     let result = request.validate();
-    assert!(result.is_err());
-    assert_eq!(result.unwrap_err(), "Query cannot be empty");
+    assert!(result.is_err(), "Should reject whitespace-only query");
 }
 
 #[test]
@@ -109,8 +108,7 @@ fn test_rerank_request_validation_empty_documents() {
     };
 
     let result = request.validate();
-    assert!(result.is_err());
-    assert_eq!(result.unwrap_err(), "Documents list cannot be empty");
+    assert!(result.is_err(), "Should reject empty documents list");
 }
 
 #[test]
@@ -126,8 +124,7 @@ fn test_rerank_request_validation_top_k_zero() {
     };
 
     let result = request.validate();
-    assert!(result.is_err());
-    assert_eq!(result.unwrap_err(), "top_k must be greater than 0");
+    assert!(result.is_err(), "Should reject top_k of zero");
 }
 
 #[test]