Core/Battlegrounds: Fix all-arenas instance lookup

[ialbahub]

Changes proposed:

• Fix BattlegroundMgr::GetBattleground lookups for BATTLEGROUND_AA. Arena instances are stored under their actual arena type, so looking only in the all-arenas container does not find them.
• Search the instance containers for all-arenas requests and accept only arena instances.
• Add a regression test for arena lookup and invited-player count cleanup through BattlegroundQueue::RemovePlayer.

Issues addressed:

Canceling an arena invitation can leave the invited-player count unchanged because the queue cannot find the arena instance. No linked issue.

Tests performed:

• Builds with Clang 18, Debug and MySQL, with dynamic scripts, no PCH and warnings treated as errors: worldserver, bnetserver and tests.
• All 36 CTests pass. The regression passes 9 assertions and fails without the lookup fix.
• Tests check arena/non-arena and wrong-type lookups, then remove an invited player and verify the invited count and queue records are cleared.

Known issues and TODO list:

• Test the invitation/cancellation flow in-game. The automated test uses an offline player and exercises the stored instance and queue methods.

Please review the all-arenas lookup, arena-only filter and invited-count cleanup.

[aquadeus]

aislop must be stopped

[ialbahub]

aislop must be stopped

? I have these for my own repo, thought I would share to fix some of the issues I ran into