server: (router) lazy-load startup_models after main setup (#27424)

* server: (router) lazy-load startup_models after main setup

* only allow is_first_load to populate it

* nits

* nits 2

tools/server/server-models.h

@@ -136,6 +136,10 @@ private:
     // if true, the next get_meta() will trigger a reload of model list
     bool need_reload = false;
 
+    // models marked with load-on-startup, unset once load_startup_models() drains it
+    // no value means the startup phase is over, so a reload must not queue anything
+    std::optional<std::vector<std::string>> startup_models{std::in_place};
+
     // conv_id -> model name that currently serves its stream session, lets the resumable stream
     // routes go straight to the owning child instead of polling every one. populated when
     // proxy_request forwards a POST carrying an X-Conversation-Id. best effort: a stale entry just
@@ -231,6 +235,9 @@ public:
     //   - if a model is not running, it will be added or updated according to the source
     void load_models();
 
+    // lazy-load startup_models, to be called after main() setup phase
+    void load_startup_models();
+
     // check if a model instance exists (thread-safe)
     bool has_model(const std::string & name);