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/PlayerbotAIConfig.h

@@ -8,11 +8,14 @@
 
 #include <mutex>
 #include <unordered_map>
+#include <set>
+#include <vector>
+#include <map>
+#include <algorithm>
+#include <string>
 
-#include "Common.h"
 #include "DBCEnums.h"
 #include "SharedDefines.h"
-#include "Talentspec.h"
 
 enum class BotCheatMask : uint32
 {
@@ -62,11 +65,11 @@ enum NewRpgStatus : int
 class PlayerbotAIConfig
 {
 public:
-    PlayerbotAIConfig(){};
-    static PlayerbotAIConfig* instance()
+    static PlayerbotAIConfig& instance()
     {
         static PlayerbotAIConfig instance;
-        return &instance;
+
+        return instance;
     }
 
     bool Initialize();
@@ -449,6 +452,16 @@ public:
     bool IsRestrictedHealerDPSMap(uint32 mapId) const;
 
     std::vector<uint32> excludedHunterPetFamilies;
+
+private:
+    PlayerbotAIConfig() = default;
+    ~PlayerbotAIConfig() = default;
+
+    PlayerbotAIConfig(const PlayerbotAIConfig&) = delete;
+    PlayerbotAIConfig& operator=(const PlayerbotAIConfig&) = delete;
+
+    PlayerbotAIConfig(PlayerbotAIConfig&&) = delete;
+    PlayerbotAIConfig& operator=(PlayerbotAIConfig&&) = delete;
 };
 
 #define sPlayerbotAIConfig PlayerbotAIConfig::instance()