fix: make register_bot_type idempotent (skip instead of panic on duplicate)

Prevents test panics when multiple strategies register the same bot type
in parallel test threads.

holdem_core/src/game/bot_factory.rs

@@ -65,10 +65,11 @@ where
 {
     let mut map = CUSTOM_FACTORIES.write().unwrap_or_else(|e| e.into_inner());
     if map.contains_key(bot_type) {
-        panic!(
-            "bot type '{}' is already registered; choose a different name",
+        log::debug!(
+            "bot type '{}' is already registered; skipping re-registration",
             bot_type
         );
+        return;
     }
     map.insert(bot_type.to_string(), Box::new(factory));
 }