test(core): add GameEngineLoadGameTest for loadGame/exportGame

Add 3 focused tests for GameEngine load/export functionality:
- loadGame with PgnParser validates PGN parsing and undo/redo state
- loadGame with FenParser validates position loading without move replay
- exportGame with PgnExporter validates PGN output

Fix: PgnParser now extends GameContextImport trait for consistency with FenParser.

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

modules/io/src/main/scala/de/nowchess/io/pgn/PgnParser.scala

@@ -3,6 +3,7 @@ package de.nowchess.io.pgn
 import de.nowchess.api.board.*
 import de.nowchess.api.move.{Move, MoveType, PromotionPiece}
 import de.nowchess.api.game.{GameContext, HistoryMove}
+import de.nowchess.io.GameContextImport
 import de.nowchess.rules.sets.DefaultRules
 
 /** A parsed PGN game containing headers and the resolved move list. */
@@ -11,7 +12,7 @@ case class PgnGame(
   moves: List[HistoryMove]
 )
 
-object PgnParser:
+object PgnParser extends GameContextImport:
 
   /** Strictly validate a PGN text.
    *  Returns Right(PgnGame) if every move token is a legal move in the evolving position.