fix(io): update GameContextExport to Scala 3 syntax and add importGameContext tests

- Update GameContextExport trait to use Scala 3 syntax (colon-based) to match GameContextImport
- Add test coverage for FenParser.importGameContext method:
  * Valid FEN string returns Right[GameContext] with correct context data
  * Invalid FEN string returns Left[String] with error message

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

modules/io/src/main/scala/de/nowchess/io/GameContextExport.scala

@@ -2,8 +2,6 @@ package de.nowchess.io
 
 import de.nowchess.api.game.GameContext
 
-trait GameContextExport {
-  
+trait GameContextExport:
+
   def exportGameContext(context: GameContext): String
-  
-}

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

@@ -131,3 +131,17 @@ class FenParserTest extends AnyFunSuite with Matchers:
 
     board shouldBe empty
 
+  test("importGameContext: valid FEN string returns Right[GameContext]"):
+    val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
+    val result = FenParser.importGameContext(fen)
+
+    result.isRight shouldBe true
+    result.fold(_ => fail("Expected Right"), ctx => ctx.turn shouldBe Color.White)
+
+  test("importGameContext: invalid FEN string returns Left[String] with error message"):
+    val invalidFen = "invalid fen string"
+    val result = FenParser.importGameContext(invalidFen)
+
+    result.isLeft shouldBe true
+    result.fold(msg => msg should include("Invalid FEN"), _ => fail("Expected Left"))
+