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

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

sgl-router/src/config/types.rs

@@ -51,6 +51,9 @@ pub struct RouterConfig {
     pub disable_circuit_breaker: bool,
     /// Health check configuration
     pub health_check: HealthCheckConfig,
+    /// Enable Inference Gateway mode (false = proxy mode, true = IGW mode)
+    #[serde(default)]
+    pub enable_igw: bool,
 }
 
 /// Routing mode configuration
@@ -323,6 +326,7 @@ impl Default for RouterConfig {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: HealthCheckConfig::default(),
+            enable_igw: false,
         }
     }
 }
@@ -377,6 +381,11 @@ impl RouterConfig {
         }
         cfg
     }
+
+    /// Check if running in IGW (Inference Gateway) mode
+    pub fn is_igw_mode(&self) -> bool {
+        self.enable_igw
+    }
 }
 
 #[cfg(test)]
@@ -456,6 +465,7 @@ mod tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let json = serde_json::to_string(&config).unwrap();
@@ -888,6 +898,7 @@ mod tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         assert!(config.mode.is_pd_mode());
@@ -944,6 +955,7 @@ mod tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         assert!(!config.mode.is_pd_mode());
@@ -996,6 +1008,7 @@ mod tests {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         assert!(config.has_service_discovery());

sgl-router/src/config/validation.rs

@@ -344,6 +344,11 @@ impl ConfigValidator {
 
     /// Validate compatibility between different configuration sections
     fn validate_compatibility(config: &RouterConfig) -> ConfigResult<()> {
+        // IGW mode is independent - skip other compatibility checks when enabled
+        if config.enable_igw {
+            return Ok(());
+        }
+
         // All policies are now supported for both router types thanks to the unified trait design
         // No mode/policy restrictions needed anymore