test: strengthen battlefield boundary coverage

src/Domain/Battlefield.php

@@ -13,7 +13,7 @@ final readonly class Battlefield
     private array $terrain;
 
     /**
-     * @param array<array-key, mixed> $terrain
+     * @param array<string, Terrain> $terrain
      */
     public function __construct(
         public int $width,
@@ -27,19 +27,11 @@ final readonly class Battlefield
         $validatedTerrain = [];
 
         foreach ($terrain as $key => $terrainType) {
-            if (!$terrainType instanceof Terrain) {
+            if (!self::isTerrain($terrainType)) {
                 throw new InvalidArgumentException("Invalid terrain value at coordinate {$key}.");
             }
 
-            if (!is_string($key) || preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
-                throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
-            }
-
-            $position = new Position((int) $matches[1], (int) $matches[2]);
-
-            if ($key !== $position->key()) {
-                throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
-            }
+            $position = self::positionFromTerrainKey($key);
 
             if (!$this->contains($position)) {
                 throw new InvalidArgumentException("Terrain coordinate {$key} is outside the battlefield.");
@@ -51,6 +43,26 @@ final readonly class Battlefield
         $this->terrain = $validatedTerrain;
     }
 
+    private static function isTerrain(mixed $terrain): bool
+    {
+        return $terrain instanceof Terrain;
+    }
+
+    private static function positionFromTerrainKey(mixed $key): Position
+    {
+        if (!is_string($key) || preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
+            throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
+        }
+
+        $position = new Position((int) $matches[1], (int) $matches[2]);
+
+        if ($key !== $position->key()) {
+            throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
+        }
+
+        return $position;
+    }
+
     public function contains(Position $position): bool
     {
         return $position->x >= 0