feat: enforce master token authentication and reasoning support

- Added strict token validation against LLM_PROXY__SERVER__AUTH_TOKENS.
- Integrated 'reasoning_content' support into providers and server responses.
- Updated AppState to carry valid auth tokens for request-time validation.

src/main.rs

@@ -113,7 +113,7 @@ async fn main() -> Result<()> {
     };
     
     // Create application state
-    let state = AppState::new(provider_manager, db_pool, rate_limit_manager, model_registry);
+    let state = AppState::new(provider_manager, db_pool, rate_limit_manager, model_registry, config.server.auth_tokens.clone());
 
     // Create application router
     let app = Router::new()