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/server/mod.rs

@@ -32,6 +32,11 @@ async fn chat_completions(
     auth: AuthenticatedClient,
     Json(request): Json<ChatCompletionRequest>,
 ) -> Result<axum::response::Response, AppError> {
+    // Validate token against configured auth tokens
+    if !state.auth_tokens.is_empty() && !state.auth_tokens.contains(&auth.token) {
+        return Err(AppError::AuthError("Invalid authentication token".to_string()));
+    }
+    
     let start_time = std::time::Instant::now();
     let client_id = auth.client_id.clone();
     let model = request.model.clone();