feat(audio): add tap handler and SFX to RoomChest

Adds _get_press_position helper and _unhandled_input tap detection to
RoomChest, wires AudioManager.play_sfx calls for chest_tap and
item_spawn events. Guards AudioManager audio load calls with Dummy
driver check so headless unit tests stay green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/unit/test_room_chest.gd

@@ -83,3 +83,21 @@ func test_receive_item_decrements_spawned_count() -> void:
 	var item: HoldableItem = chest.get_spawned_item(0)
 	chest.receive_item(item)
 	assert_eq(chest.get_spawned_count(), 2)
+
+
+func test_get_press_position_returns_position_for_screen_touch_pressed() -> void:
+	var chest: RoomChest = RoomChest.new()
+	add_child_autofree(chest)
+	var event: InputEventScreenTouch = InputEventScreenTouch.new()
+	event.pressed = true
+	event.position = Vector2(100.0, 200.0)
+	assert_eq(chest._get_press_position(event), Vector2(100.0, 200.0))
+
+
+func test_get_press_position_returns_inf_for_screen_touch_released() -> void:
+	var chest: RoomChest = RoomChest.new()
+	add_child_autofree(chest)
+	var event: InputEventScreenTouch = InputEventScreenTouch.new()
+	event.pressed = false
+	event.position = Vector2(100.0, 200.0)
+	assert_eq(chest._get_press_position(event), Vector2.INF)