Fix bot leader handling (#2462)

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## Pull Request Description
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With the default formation, FollowAction::isUseful resolved the follow
target as "group leader" and returned false when the bot itself was
leader, so an alt bot left as group leader never moved. Gate on the
master first (matching Execute, which already positions relative to
master) and fall back to group leader only when no master is set.

Fixes [#2443](https://github.com/Noscopezz/mod-playerbots/issues/2443)


## Feature Evaluation
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1. Be leader and have alt bot in group
2. Change to Alt
3. Leader and every other bot should follow you and listen to your
commands even if you are not leader. give leader command will still work
without breaking other Fixes so we have middle groud which should make
everyone happy.
4. Play around, it should work every time


## Impact Assessment
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- Does this change increase per-bot/per-tick processing or risk scaling
poorly with thousands of bots?
    - - [x] No, not at all
    - - [ ] Minimal impact (**explain below**)
    - - [ ] Moderate impact (**explain below**)



- Does this change modify default bot behavior?
    - - [ ] No
    - - [x] Yes (**explain why**)
    explained above


- Does this change add new decision branches or increase maintenance
complexity?
    - - [x] No
    - - [ ] Yes (**explain below**)



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## Final Checklist

- - [x] Stability is not compromised.
- - [x] Performance impact is understood, tested, and acceptable.
- - [x] Added logic complexity is justified and explained.
- - [x] Any new bot dialogue lines are translated.
- - [x] Documentation updated if needed (Conf comments, WiKi commands).

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src/Ai/Base/Actions/FollowActions.cpp

@@ -259,6 +259,8 @@ bool FollowAction::isUseful()
     Unit* fTarget = nullptr;
     if (!target.empty())
         fTarget = AI_VALUE(Unit*, target);
+    else if (botAI->GetMaster())
+        fTarget = botAI->GetMaster();
     else
         fTarget = AI_VALUE(Unit*, "group leader");