refactor(WG): read core PlayersInWar instead of parallel bucket

[Nyeriah]

Summary

• Drop the module-owned _wgWarPlayers bucket in CFBG_Battlefield. WG balance and end-of-war fake cleanup now read core's Battlefield::GetPlayersInWarSet(team) directly.
• Remove OnBattlefieldPlayerLeaveWar and OnBattlefieldPlayerLeaveZone subscriptions — they existed only to drain the parallel bucket.

Why

The bucket mirrored Battlefield::PlayersInWar but had its own lifecycle, which opened a real divergence race: OnBattlefieldPlayerJoinWar fires before core's PlayersInWar.insert, and if the subsequent AddOrSetPlayerToCorrectBfGroup call rejected the join, the module had already inserted into _wgWarPlayers (and morphed the player) while core had not. Reading GetPlayersInWarSet directly eliminates that window — the candidate is genuinely in neither side at hook time, and we only mutate module state if core actually commits the join afterward.

Hook-timing notes preserved in code comments:

• OnBattlefieldPlayerJoinWar fires before PlayersInWar.insert, so the new GetPlayersInWarSet reads give the same pre-candidate snapshot the bucket used to.
• OnBattlefieldWarEnd fires before OnBattleEnd clears PlayersInWar (paired core change), so the war set is still iterable for fake cleanup.

⚠️ Paired core dependency

This PR requires the paired AzerothCore PR azerothcore/azerothcore-wotlk#26030, which reorders sScriptMgr->OnBattlefieldWarEnd(...) to fire before OnBattleEnd(...) in Battlefield::EndBattle. Without it, the new OnBattlefieldWarEnd body reads an empty PlayersInWar and fake players never get cleaned up at war end.

Do not merge this until azerothcore/azerothcore-wotlk#26030 is merged.

Test plan

• Build cleanly against the paired core change.
• Wintergrasp starts; balance gate at OnBattlefieldPlayerJoinWar continues to flip Alliance/Horde candidates when one side is heavier (verify with .bf list before/after a few accepts).
• At war end, every faked WG participant has their real race/team/faction restored (check with .cfbg debug <name> immediately after the war ends and on a player who stays in the zone).
• A player who logs out mid-war and reconnects after war end is restored on relog (existing OnPlayerLogout path, unchanged).
• A player who walks out of the WG zone during war still has fake cleared at zone change (OnPlayerUpdateZone path, unchanged).

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