Improper singletons migration to clean Meyer's singletons (cherry-pick) (#2082)

# Pull Request

- Applies the clean and corrected singletons, Meyer pattern. (cherry
picked from @SmashingQuasar )

Testing by just playing the game in various ways. Been tested by myself
@Celandriel and @SmashingQuasar
---

## Complexity & Impact

- Does this change add new decision branches?
    - [x] No
    - [ ] Yes (**explain below**)

- Does this change increase per-bot or per-tick processing?
    - [x] No
    - [ ] Yes (**describe and justify impact**)

- Could this logic scale poorly under load?
    - [x] No
    - [ ] Yes (**explain why**)

---

## Defaults & Configuration

- Does this change modify default bot behavior?
    - [x] No
    - [ ] Yes (**explain why**)

---

## AI Assistance

- Was AI assistance (e.g. ChatGPT or similar tools) used while working
on this change?
    - [x] No
    - [ ] Yes (**explain below**)
---

## Final Checklist

- [x] Stability is not compromised
- [x] Performance impact is understood, tested, and acceptable
- [x] Added logic complexity is justified and explained
- [x] Documentation updated if needed

---

## Notes for Reviewers

Anything that significantly improves realism at the cost of stability or
performance should be carefully discussed
before merging.

---------

Co-authored-by: Nicolas Lebacq <nicolas.cordier@outlook.com>
Co-authored-by: Keleborn <22352763+Celandriel@users.noreply.github.com>

src/Db/PlayerbotDungeonRepository.h

@@ -6,10 +6,9 @@
 #ifndef _PLAYERBOT_PLAYERBOTDUNGEONREPOSITORY_H
 #define _PLAYERBOT_PLAYERBOTDUNGEONREPOSITORY_H
 
-#include <map>
 #include <vector>
+#include <string>
 
-#include "Common.h"
 #include "DBCEnums.h"
 
 struct DungeonSuggestion
@@ -22,24 +21,31 @@ struct DungeonSuggestion
     std::string strategy;
 };
 
+// @TODO: Completely unused at this moment.
 class PlayerbotDungeonRepository
 {
 public:
-    PlayerbotDungeonRepository(){};
-    ~PlayerbotDungeonRepository(){};
-    static PlayerbotDungeonRepository* instance()
+    static PlayerbotDungeonRepository& instance()
     {
         static PlayerbotDungeonRepository instance;
-        return &instance;
+
+        return instance;
     }
 
     void LoadDungeonSuggestions();
     std::vector<DungeonSuggestion> const GetDungeonSuggestions();
 
 private:
+    PlayerbotDungeonRepository() = default;
+    ~PlayerbotDungeonRepository() = default;
+
+    PlayerbotDungeonRepository(const PlayerbotDungeonRepository&) = delete;
+    PlayerbotDungeonRepository& operator=(const PlayerbotDungeonRepository&) = delete;
+
+    PlayerbotDungeonRepository(PlayerbotDungeonRepository&&) = delete;
+    PlayerbotDungeonRepository& operator=(PlayerbotDungeonRepository&&) = delete;
+
     std::vector<DungeonSuggestion> m_dungeonSuggestions;
 };
 
-#define sPlayerbotDungeonRepository PlayerbotDungeonRepository::instance()
-
 #endif