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/Ai/Base/Actions/AcceptInvitationAction.cpp

@@ -46,10 +46,10 @@ bool AcceptInvitationAction::Execute(Event event)
     if (!bot->GetGroup() || !bot->GetGroup()->IsMember(inviter->GetGUID()))
         return false;
 
-    if (sRandomPlayerbotMgr->IsRandomBot(bot))
+    if (sRandomPlayerbotMgr.IsRandomBot(bot))
         botAI->SetMaster(inviter);
     // else
-    // sPlayerbotRepository->Save(botAI);
+    // PlayerbotRepository::instance().Save(botAI);
 
     botAI->ResetStrategies();
     botAI->ChangeStrategy("+follow,-lfg,-bg", BOT_STATE_NON_COMBAT);
@@ -57,7 +57,7 @@ bool AcceptInvitationAction::Execute(Event event)
 
     botAI->TellMaster("Hello");
 
-    if (sPlayerbotAIConfig->summonWhenGroup && bot->GetDistance(inviter) > sPlayerbotAIConfig->sightDistance)
+    if (sPlayerbotAIConfig.summonWhenGroup && bot->GetDistance(inviter) > sPlayerbotAIConfig.sightDistance)
     {
         Teleport(inviter, bot, true);
     }