Fixes #558

This PR updates the validateFen() function to correctly handle cases where the side to move is already delivering check.

Changes

• Updated validateFen() logic to detect when the side to move is already delivering check

export function validateFen(fen: string): { ok: boolean; error?: string } {
  
  // .. Rest of code

  // 12th criterion: is side to move already delivering check?
  const tokensTemp = tokens.slice()
  tokensTemp[1] = swapColor(tokens[1] as Color)
  const tempFen = tokensTemp.join(' ') // swap the side to move

  const tempChess = new Chess(tempFen, { skipValidation: true })
  if (tempChess.inCheck()) {
    return {
      ok: false,
      error: 'Invalid FEN: side to move is already delivering check',
    }
  }

  return { ok: true }
}

• Added and updated related test cases
  • Two testcases were added in validate_fen.test.ts.
  • Four testcases in put.test.ts were updated because they triggered the invalid FEN error. Please check if the modified testcases still reflect the original intention.
    put - occupying white en passant square clears it
put - replacing black pawn clears white en passant square 2
put - occupying black en passant square clears it
put - replacing white pawn clears black en passant square 2