fix(npc): don't respawn an NPC which was disposed while waiting to respawn

[ezequielzacca]

Problem

A killed NPC schedules its respawn (Task.Delay(RespawnDelay) → RespawnAsync). When the NPC is disposed while it waits — e.g. its spawn area is deleted in the admin panel (the MapInitializer delete handler disposes every NPC of the area, dead ones included), or its map/mini game is torn down — the pending task still calls Initialize() and GameMap.RespawnAsync, putting the disposed object back into the map's area of interest manager. In game it shows up as a monster which can't be attacked and stays until the server restarts; its object id may already be reused.

The check needed for this didn't work either: AsyncDisposable.DisposeAsync called Dispose(true) directly, so IsDisposed stayed false after an async dispose (and a second DisposeAsync released the resources again).

Changes

• AttackableNpcBase.RespawnAsync returns early when the NPC is disposed or disposing.
• AsyncDisposable.DisposeAsync calls Dispose(), which disposes the managed resources and sets IsDisposed.
• Tests: a killed monster disposed before its respawn delay doesn't come back (and a killed, not disposed one still respawns); IsDisposed is set after DisposeAsync, and a second DisposeAsync doesn't dispose again.

Note: since IsDisposed is now true after DisposeAsync, existing IsDisposed checks on AsyncDisposable subclasses (e.g. in MiniGameManager, BotManager) now see disposed instances, which seems to be what they intended.

Testing

• All test projects pass (MUnique.OpenMU.Tests 1897, Persistence.Initialization, Network, Web, PlugIns, ...).
• Verified in game: killed the monsters of a spawn area, deleted the area right away; before the fix they reappeared after their respawn delay, now none come back.

[sven-n]

Review: looks good. You found and fixed the AsyncDisposable root cause, not just the symptom.

Verified locally (head a0d72b4, merged with current master):

• src/Startup builds with 0 errors.
• MUnique.OpenMU.Tests: 1897/1897 pass, including MonsterRespawnTest and AsyncDisposableTest.
• MUnique.OpenMU.Web.Tests: 278/278 pass.

Notes:

• DisposeAsync → Dispose() works with the two separate IsDisposing flags: AsyncDisposable shadows IsDisposing with its own field, so the base Dispose() still runs Dispose(true) once and then sets IsDisposed. A second DisposeAsync is now a no-op, as intended.
• I looked at the existing IsDisposed checks that now see true after an async dispose. BotManager, MiniGameStartBasePlugIn, MagicEffectsList and InvasionGameServerState all treat it as "skip". MagicEffect.ResetTimer throws on a disposed effect, but its only caller (MagicEffectsList) already checks IsDisposed || IsDisposing first. So nothing starts throwing.
• The early return in RespawnAsync covers both "disposed" and "disposing", which also covers a teardown that's still running.

Merging.

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