Fix arity and return value mismatches

`assert` returns every argument it is given, so `return assert(x, msg)`
was leaking the message as a second return value.

`get_tile` is documented as taking x and y. The async function class
declared `@operator call` with no parameters, which made every async
call look over supplied.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_scenario/module/control/degrading_tiles.lua

@@ -18,7 +18,7 @@ end
 --- @param surface LuaSurface
 --- @param position MapPosition.struct
 local function degrade_tile(surface, position)
-    local tile = surface.get_tile(position)
+    local tile = surface.get_tile(position.x, position.y)
     local tile_name = tile.name
     local degrade_tile_name = config.degrade_order[tile_name]
     if not degrade_tile_name then return end
@@ -61,7 +61,7 @@ end
 --- @param position MapPosition.struct
 --- @return number?
 local function get_tile_strength(surface, position)
-    local tile = surface.get_tile(position)
+    local tile = surface.get_tile(position.x, position.y)
     local tile_name = tile.name
     local strength = config.strengths[tile_name]
     if not strength then return end

exp_scenario/module/control/spawn_area.lua

@@ -166,7 +166,7 @@ local function clear_spawn_area(surface, offset)
     local get_tile = surface.get_tile
 
     -- Make sure a non water tile is used for filling
-    local starting_tile = get_tile(offset)
+    local starting_tile = get_tile(offset.x, offset.y)
     local fill_tile = starting_tile.collides_with("player") and "landfill" or starting_tile.name
     local fill_radius = config.spawn_area.landfill_radius
     local fill_radius_sqr = fill_radius ^ 2
@@ -186,7 +186,7 @@ local function clear_spawn_area(surface, offset)
             if dst < tile_radius_sqr then
                 -- If it is inside the decon radius always set the tile
                 tiles_to_make[#tiles_to_make + 1] = { name = decon_tile, position = pos }
-            elseif dst < fill_radius_sqr and get_tile(pos).collides_with("player") then
+            elseif dst < fill_radius_sqr and get_tile(pos.x, pos.y).collides_with("player") then
                 -- If it is inside the fill radius only set the tile if it is water
                 tiles_to_make[#tiles_to_make + 1] = { name = fill_tile, position = pos }
             end