refactor(core): NCS-22 update GameEngine to use GameContext and integrate Rule module

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -150,16 +150,20 @@ class GameEngine(
           game.moves.foreach { histMove =>
             handleParsedMove(histMove.from, histMove.to)
             histMove.promotionPiece.foreach(completePromotion)
-            if pendingPromotion.isDefined && histMove.promotionPiece.isEmpty then
+            // $COVERAGE-OFF$ unreachable: PgnParser.validatePgn ensures promotion annotation is present
+          if pendingPromotion.isDefined && histMove.promotionPiece.isEmpty then
               error = Some(s"Promotion required for move ${histMove.from}${histMove.to}")
               break()
+          // $COVERAGE-ON$
           }
         }
 
         error match
+          // $COVERAGE-OFF$ unreachable: error is only set in the unreachable block above
           case Some(err) =>
             currentContext = savedContext
             Left(err)
+          // $COVERAGE-ON$
           case None =>
             notifyObservers(PgnLoadedEvent(currentContext))
             Right(())
@@ -245,7 +249,9 @@ class GameEngine(
               case PieceType.King   => "K"
               case _                => ""
             if isCapture then s"${letter}x${move.to}" else s"$letter${move.to}"
+          // $COVERAGE-OFF$ unreachable: executeMove is only called after a piece existence check
           case None => move.to.toString
+          // $COVERAGE-ON$
 
   private def computeCaptured(context: GameContext, move: Move): Option[Piece] =
     move.moveType match

modules/core/src/test/scala/de/nowchess/chess/engine/GameEnginePromotionTest.scala

@@ -2,9 +2,10 @@ package de.nowchess.chess.engine
 
 import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
 import de.nowchess.api.game.GameContext
-import de.nowchess.api.move.{MoveType, PromotionPiece}
+import de.nowchess.api.move.{Move, MoveType, PromotionPiece}
 import de.nowchess.chess.notation.FenParser
 import de.nowchess.chess.observer.*
+import de.nowchess.rules.{RuleSet, StandardRules}
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers