Clear game handler DBC caches on expansion switch

Add resetDbcCaches() to GameHandler to clear stale spell name, skill,
taxi, and talent caches when switching expansions. Called from
reloadExpansionData() so switching servers (e.g. WotLK to Turtle)
reloads DBC data instead of using stale entries from the previous
expansion.

include/game/game_handler.hpp

@@ -800,6 +800,12 @@ public:
      */
     void update(float deltaTime);
 
+    /**
+     * Reset DBC-backed caches so they reload from new expansion data.
+     * Called by Application when the expansion profile changes.
+     */
+    void resetDbcCaches();
+
 private:
     void autoTargetAttacker(uint64_t attackerGuid);