refactor(io): change GameContextImport return type from Option to Either

Update GameContextImport trait to return Either[String, GameContext] instead of
Option[GameContext] to provide better error context during parsing. Update FenParser
implementation to convert Option to Either using toRight().

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

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

@@ -2,8 +2,6 @@ package de.nowchess.io
 
 import de.nowchess.api.game.GameContext
 
-trait GameContextImport {
+trait GameContextImport:
 
-  def importGameContext(input: String): Option[GameContext]
-
-}
+  def importGameContext(input: String): Either[String, GameContext]

modules/io/src/main/scala/de/nowchess/io/fen/FenParser.scala

@@ -28,7 +28,8 @@ object FenParser extends GameContextImport:
         moves = List.empty
       )
 
-  def importGameContext(input: String): Option[GameContext] = parseFen(input)
+  def importGameContext(input: String): Either[String, GameContext] =
+    parseFen(input).toRight("Invalid FEN string")
 
   /** Parse active color ("w" or "b"). */
   private def parseColor(s: String): Option[Color] =