Core/Battlegrounds: Identify arena types from BattlemasterList - #32052

Changes proposed:

• Use BattlemasterList data in BattlegroundMgr::IsArenaType instead of the hardcoded arena list.
• Keep the BATTLEGROUND_AA special case and return false when the client record is missing.
• Add a regression test showing that an arena outside the old list receives client instance ID 0, while an ordinary battleground receives ID 1.

Issues addressed:

Arena types outside the hardcoded list are treated as ordinary battlegrounds when allocating client instance IDs. No linked issue.

Tests performed:

• Builds with Clang 18, Release and MySQL, with warnings treated as errors: worldserver, bnetserver and tests.
• All 36 CTests pass. The regression passes 4 assertions and fails without the fix.
• Checked arena and battleground records outside the old list, as well as BATTLEGROUND_AA.

Known issues and TODO list:

• Verify arena queue behavior in-game. The automated test covers classification through the client instance ID allocator.

Please review the use of BattlemasterList and the all-arenas special case.