[router] Add IGW (Inference Gateway) Feature Flag (#9371)

Co-authored-by: Yineng Zhang <me@zhyncs.com>

sgl-router/tests/api_endpoints_test.rs

@@ -51,6 +51,7 @@ impl TestContext {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         Self::new_with_config(config, worker_configs).await
@@ -1093,6 +1094,7 @@ mod error_tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let ctx = TestContext::new_with_config(
@@ -1444,6 +1446,7 @@ mod pd_mode_tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         // Create app context
@@ -1599,6 +1602,7 @@ mod request_id_tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let ctx = TestContext::new_with_config(

sgl-router/tests/request_formats_test.rs

@@ -42,6 +42,7 @@ impl TestContext {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let mut workers = Vec::new();

sgl-router/tests/streaming_tests.rs

@@ -43,6 +43,7 @@ impl TestContext {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let mut workers = Vec::new();

sgl-router/tests/test_pd_routing.rs

@@ -184,6 +184,7 @@ mod test_pd_routing {
                 disable_retries: false,
                 disable_circuit_breaker: false,
                 health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+                enable_igw: false,
             };
 
             // Router creation will fail due to health checks, but config should be valid