fix: digging up a treasure map chest no longer crashes when guardians spawn (#2692)

## Summary
Digging up a decoded treasure map of level 0, or of level 2 and above, throws inside `DigTimer.OnTick` once the chest surfaces. The exception escapes the timer, so the server goes down:

```
System.NullReferenceException: Object reference not set to an instance of an object.
   at Server.Items.TreasureMap.DigTimer.OnTick() in ...\Items\Maps\TreasureMap.cs:line 809
```

#2568 made `TreasureMapChest._guardians` lazily created: the constructor no longer initialises it, and migration nulls an empty list. #2569 updated the read in `CheckLocked` to match, but the dig timer still adds each spawned guardian directly:

```csharp
if (bc != null)
{
    m_Chest.Guardians.Add(bc); // null on a freshly dug chest
}
```

Level 1 maps survive only because they spawn no guardians. I confirmed the regression by running the dig test below at #2568's parent, where it passes, and at #2568, where it fails.

The dig timer now calls the generated `AddToGuardians`, which creates the list on first use.

## Testing
- `dotnet build`: 0 warnings, 0 errors.
- New `TreasureMapChestGuardianTests`:
  - `Dig_AddsSpawnedGuardiansToTheChest` (levels 0 and 2) drives the real `DigTimer` tick by tick until the chest surfaces. Before the fix both cases fail with the `NullReferenceException` above. It needs client map data, so it calls `TileDataRequirement.SkipIfMissing()` and skips on CI.
  - `AddToGuardians_OnNewChest_KeepsLevelZeroChestShut` needs no client data, so it runs on CI: the list starts null, the first guardian creates it, and a level 0 chest stays shut to players until that guardian is dead.
- Full `UOContent.Tests`: 1196 passed and 2 skipped with client data; 1105 passed and 93 skipped without.

Projects/UOContent/Items/Maps/TreasureMap.cs

@@ -806,7 +806,7 @@ public partial class TreasureMap : MapItem
 
                     if (bc != null)
                     {
-                        m_Chest.Guardians.Add(bc);
+                        m_Chest.AddToGuardians(bc);
                     }
                 }
             }