fix(io): update FEN tests to use Either API after parseFen signature change

FenParser.parseFen now returns Either[String, GameContext] instead of Option.
Updated all tests to use Either combinators (isRight, isLeft, fold) instead
of Option methods (isDefined, get). Both FenParserTest and FenExporterTest
now properly handle the Either-based return type.

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

modules/io/src/test/scala/de/nowchess/io/fen/FenExporterTest.scala

@@ -97,6 +97,6 @@ class FenExporterTest extends AnyFunSuite with Matchers:
     )
     val fen = FenExporter.gameContextToFen(gameContext)
     FenParser.parseFen(fen) match
-      case Some(ctx) => ctx.halfMoveClock shouldBe 42
+      case Right(ctx) => ctx.halfMoveClock shouldBe 42
-      case None     => fail("FEN parsing failed")
+      case Left(err)  => fail(s"FEN parsing failed: $err")
 

modules/io/src/test/scala/de/nowchess/io/fen/FenParserTest.scala

@@ -66,49 +66,55 @@ class FenParserTest extends AnyFunSuite with Matchers:
     val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
     val context = FenParser.parseFen(fen)
 
-    context.isDefined shouldBe true
+    context.isRight shouldBe true
-    context.get.turn shouldBe Color.White
+    context.fold(_ => fail(), ctx =>
-    context.get.castlingRights.whiteKingSide shouldBe true
+      ctx.turn shouldBe Color.White
-    context.get.castlingRights.whiteQueenSide shouldBe true
+      ctx.castlingRights.whiteKingSide shouldBe true
-    context.get.castlingRights.blackKingSide shouldBe true
+      ctx.castlingRights.whiteQueenSide shouldBe true
-    context.get.castlingRights.blackQueenSide shouldBe true
+      ctx.castlingRights.blackKingSide shouldBe true
-    context.get.enPassantSquare shouldBe None
+      ctx.castlingRights.blackQueenSide shouldBe true
-    context.get.halfMoveClock shouldBe 0
+      ctx.enPassantSquare shouldBe None
+      ctx.halfMoveClock shouldBe 0
+    )
 
   test("parse full FEN - after e4"):
     val fen = "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1"
     val context = FenParser.parseFen(fen)
 
-    context.get.turn shouldBe Color.Black
+    context.fold(_ => fail(), ctx =>
-    context.get.enPassantSquare shouldBe Some(Square(File.E, Rank.R3))
+      ctx.turn shouldBe Color.Black
+      ctx.enPassantSquare shouldBe Some(Square(File.E, Rank.R3))
+    )
 
   test("parse full FEN - invalid parts count"):
     val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq"
     val context = FenParser.parseFen(fen)
 
-    context.isDefined shouldBe false
+    context.isLeft shouldBe true
 
   test("parse full FEN - invalid color"):
     val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR x KQkq - 0 1"
     val context = FenParser.parseFen(fen)
 
-    context.isDefined shouldBe false
+    context.isLeft shouldBe true
 
   test("parse full FEN - invalid castling"):
     val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w XYZ - 0 1"
     val context = FenParser.parseFen(fen)
 
-    context.isDefined shouldBe false
+    context.isLeft shouldBe true
 
   test("parseFen: castling '-' produces no castling rights"):
     val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w - - 0 1"
     val context = FenParser.parseFen(fen)
 
-    context.isDefined shouldBe true
+    context.isRight shouldBe true
-    context.get.castlingRights.whiteKingSide shouldBe false
+    context.fold(_ => fail(), ctx =>
-    context.get.castlingRights.whiteQueenSide shouldBe false
+      ctx.castlingRights.whiteKingSide shouldBe false
-    context.get.castlingRights.blackKingSide shouldBe false
+      ctx.castlingRights.whiteQueenSide shouldBe false
-    context.get.castlingRights.blackQueenSide shouldBe false
+      ctx.castlingRights.blackKingSide shouldBe false
+      ctx.castlingRights.blackQueenSide shouldBe false
+    )
 
   test("parseBoard: returns None when a rank has too many files (overflow beyond 8)"):
     // "9" alone would advance fileIdx to 9, exceeding 8 → None