Add comprehensive test suite for game mechanics and level handling

- Introduced `test_final_level_flow.py` to validate final level transitions and game end scenarios.
- Created `test_game_over_flow.py` to ensure game over conditions trigger correctly based on rat counts.
- Implemented `test_keybindings.py` to verify keybinding configurations and their context-specific actions.
- Developed `test_level_editor.py` to assess level editor functionalities and layout computations.
- Added `test_level_io.py` for testing level data serialization and deserialization.
- Established `test_loop_logic_parity.py` to ensure consistent game state across multiple simulation runs.
- Created `test_non_regression.py` to simulate game behavior and capture states for future verification.
- Implemented `test_verify.py` to compare current game states against a golden master for regression detection.

units/gas.py

@@ -1,5 +1,4 @@
-from .unit import Unit
-from .rat import Rat
+from .unit import Unit, UnitType
 from engine.collision_system import CollisionLayer
 import random
 
@@ -8,7 +7,7 @@ AGE_THRESHOLD = 200
 
 class Gas(Unit):
     def __init__(self, game, position=(0,0), id=None, parent_id=None):
-        super().__init__(game, position, id, collision_layer=CollisionLayer.GAS)
+        super().__init__(game, position, id, collision_layer=CollisionLayer.GAS, unit_type=UnitType.GAS)
         self.parent_id = parent_id
         # Specific attributes for gas
         self.speed = 50
@@ -33,7 +32,7 @@ class Gas(Unit):
         
         for victim_id in victim_ids:
             victim = self.game.unit_manager.get_unit_by_id(victim_id)
-            if victim and isinstance(victim, Rat):
+            if victim and victim.type in [UnitType.RAT_MALE, UnitType.RAT_FEMALE]:
                 if victim.partial_move > 0.5:
                     victim.gassed += 1
         
@@ -44,7 +43,7 @@ class Gas(Unit):
         
         for victim_id in victim_ids_before:
             victim = self.game.unit_manager.get_unit_by_id(victim_id)
-            if victim and isinstance(victim, Rat):
+            if victim and victim.type in [UnitType.RAT_MALE, UnitType.RAT_FEMALE]:
                 if victim.partial_move < 0.5:
                     victim.gassed += 1
         
@@ -59,7 +58,7 @@ class Gas(Unit):
                 new_x = self.position[0] + dx
                 new_y = self.position[1] + dy
                 if not self.game.map.is_wall(new_x, new_y):
-                    if not any(isinstance(unit, Gas) for unit in self.game.units.values() if unit.position == (new_x, new_y)):
+                    if not any(unit.type == UnitType.GAS for unit in self.game.units.values() if unit.position == (new_x, new_y)):
                         print(f"Spreading gas from {self.position} to ({new_x}, {new_y})")
                         self.game.unit_manager.spawn_unit(Gas, (new_x, new_y), parent_id=self.parent_id if self.parent_id else self.id)
     def collisions(self):