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.cpp

@@ -660,25 +660,25 @@ bool PlayerbotAIConfig::Initialize()
     }
 
     // Assign account types after accounts are created
-    sRandomPlayerbotMgr->AssignAccountTypes();
+    sRandomPlayerbotMgr.AssignAccountTypes();
 
-    if (sPlayerbotAIConfig->enabled)
+    if (sPlayerbotAIConfig.enabled)
     {
-        sRandomPlayerbotMgr->Init();
+        sRandomPlayerbotMgr.Init();
     }
 
-    sPlayerbotGuildMgr->Init();
-    sRandomItemMgr->Init();
-    sRandomItemMgr->InitAfterAhBot();
-    sPlayerbotTextMgr->LoadBotTexts();
-    sPlayerbotTextMgr->LoadBotTextChance();
+    PlayerbotGuildMgr::instance().Init();
+    sRandomItemMgr.Init();
+    sRandomItemMgr.InitAfterAhBot();
+    PlayerbotTextMgr::instance().LoadBotTexts();
+    PlayerbotTextMgr::instance().LoadBotTextChance();
     PlayerbotFactory::Init();
 
     AiObjectContext::BuildAllSharedContexts();
 
-    if (sPlayerbotAIConfig->randomBotSuggestDungeons)
+    if (sPlayerbotAIConfig.randomBotSuggestDungeons)
     {
-        sPlayerbotDungeonRepository->LoadDungeonSuggestions();
+        PlayerbotDungeonRepository::instance().LoadDungeonSuggestions();
     }
 
     excludedHunterPetFamilies.clear();