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/CheckMailAction.cpp

@@ -28,7 +28,7 @@ bool CheckMailAction::Execute(Event event)
             continue;
 
         uint32 account = owner->GetSession()->GetAccountId();
-        if (sPlayerbotAIConfig->IsInRandomAccountList(account))
+        if (sPlayerbotAIConfig.IsInRandomAccountList(account))
             continue;
 
         ProcessMail(mail, owner, trans);
@@ -80,7 +80,7 @@ void CheckMailAction::ProcessMail(Mail* mail, Player* owner, CharacterDatabaseTr
         if (!item)
             continue;
 
-        if (!sGuildTaskMgr->CheckItemTask(i->item_template, item->GetCount(), owner, bot, true))
+        if (!GuildTaskMgr::instance().CheckItemTask(i->item_template, item->GetCount(), owner, bot, true))
         {
             std::ostringstream body;
             body << "Hello, " << owner->GetName() << ",\n";