fix(BG): clear stale WG fake at BG entry to prevent team desync

[Nyeriah]

Summary

• A player faked for Wintergrasp who joins a BG retains the WG fake into the BG instance: OnBattlefieldPlayerLeaveZone stopped clearing it in fix(WG): defer ClearFakePlayer to OnPlayerUpdateZone #147, and the deferred OnPlayerUpdateZone clear is blocked by the InBattleground() guard added alongside fix: regression introduced in my previous commit + add guard #148 once the teleport lands on the BG map.
• The persistent fake short-circuits both BalanceTeamsOnEntry (its IsPlayerFake guard) and SetFakeRaceAndMorph (same guard), and can desync bgTeamId from GetTeamId(). The worst case: a real Alliance player queues for a BG before WG war pops, gets faked Horde mid-queue, and lands on the Alliance queue-side spawn with a Horde faction/visual — flag captures, faction checks, and graveyard assignment all disagree.
• Fix: clear any pre-existing fake at the top of ValidatePlayerForBG so the BG entry path re-evaluates against bgTeamId. The fix(WG): defer ClearFakePlayer to OnPlayerUpdateZone #147 timing concern doesn't apply here — Battlefield::HandlePlayerLeaveZone has already finished by the time Battleground::AddPlayer runs on the new map.

Test plan

• Real Alliance player queues for a BG before WG war pops, gets WG-faked Horde mid-queue, accepts the BG invite — confirm they land on the queue-side as their real Alliance race with a consistent faction (no Horde-on-Alliance desync).
• WG-faked Horde player queues for a BG (so queue side = Horde), accepts the BG invite — confirm a fresh BG fake is applied (race randomized for the BG, SETTING_CFBG_RACE honored if set), bgTeamId matches GetTeamId().
• Normal BG entry from outside WG still works (no double-fake, no missing fake).
• BG end / leave correctly restores real race / faction (no leftover fake from the entry-time clear).
• WG entry from outside (no BG involvement) still fakes and restores correctly — no regression on the WG-only path.
• BalanceTeamsOnEntry disabled (= 0): the fix still resolves the desync; player just doesn't get rebalanced (expected).

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