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/Util/ServerFacade.h

@@ -6,8 +6,6 @@
 #ifndef _PLAYERBOT_SERVERFACADE_H
 #define _PLAYERBOT_SERVERFACADE_H
 
-#include "Common.h"
-
 class Player;
 class Unit;
 class WorldObject;
@@ -29,12 +27,13 @@ public:
     /**
      * @brief Get singleton instance.
      *
-     * @return ServerFacade* Pointer to the singleton instance.
+     * @return ServerFacade& Reference to the singleton instance.
      */
-    static ServerFacade* instance()
+    static ServerFacade& instance()
     {
         static ServerFacade instance;
-        return &instance;
+
+        return instance;
     }
 
 public:
@@ -123,7 +122,4 @@ public:
     void SendPacket(Player* player, WorldPacket* packet);
 };
 
-/** Global singleton accessor. */
-#define sServerFacade ServerFacade::instance()
-
 #endif