server: minor tweaks to use more cpp features (#23785)

* misc(server): add default port to impl RAII

* misc(server): register_gcp_compat() can be const

* misc(server): use proper cpp const/auto methods

* misc(server): do not reset a unique_ptr, use make_unique instead to be exception safe

tools/server/server-http.h

@@ -73,7 +73,7 @@ struct server_http_context {
 
     std::string path_prefix;
     std::string hostname;
-    int port;
+    int port    = 8080;
     bool is_ssl = false;
 
     server_http_context();
@@ -88,7 +88,7 @@ struct server_http_context {
 
     // Register the Google Cloud Platform (Vertex AI) compat (AIP_PREDICT_ROUTE env var, or /predict)
     // Must be called AFTER all other API routes are registered
-    void register_gcp_compat();
+    void register_gcp_compat() const;
 
     // for debugging
     std::string listening_address;