refactor: use named constants for test cases to enable us maintain the project easily

src/World.Net.UnitTests/Countries/AlandIslandsTest.cs

@@ -2,6 +2,15 @@
 
 public sealed class AlandIslandsTest
 {
+    private const string ALAND_ISLANDS_NAME = "Aland Islands";
+    private const string ALAND_ISLANDS_OFFICIAL_NAME = "Åland Islands";
+    private const string ALAND_ISLANDS_NATIVE_NAME = "Åland";
+    private const string ALAND_ISLANDS_CAPITAL = "Mariehamn";
+    private const int ALAND_ISLANDS_NUMERIC_CODE = 248;
+    private const string ALAND_ISLANDS_ISO2_CODE = "AX";
+    private const string ALAND_ISLANDS_ISO3_CODE = "ALA";
+    private const string ALAND_ISLANDS_CALLING_CODE = "+358";
+
     [Fact]
     public void GetCountry_ReturnsCorrectInformation_ForAlandIslands()
     {
@@ -12,14 +21,14 @@ public void GetCountry_ReturnsCorrectInformation_ForAlandIslands()
         var country = CountryProvider.GetCountry(existingCountryId);
 
         //Assert
-        Assert.Equal(2, country.Id);
-        Assert.Equal("Aland Islands", country.Name);
-        Assert.Equal("Åland Islands", country.OfficialName);
-        Assert.Equal("Åland", country.NativeName);
-        Assert.Equal("Mariehamn", country.Capital);
-        Assert.Equal(248, country.NumericCode);
-        Assert.Equal("AX", country.ISO2Code);
-        Assert.Equal("ALA", country.ISO3Code);
-        Assert.Equal("+358", country.CallingCode);
+        Assert.Equal(existingCountryId, country.Id);
+        Assert.Equal(ALAND_ISLANDS_NAME, country.Name);
+        Assert.Equal(ALAND_ISLANDS_OFFICIAL_NAME, country.OfficialName);
+        Assert.Equal(ALAND_ISLANDS_NATIVE_NAME, country.NativeName);
+        Assert.Equal(ALAND_ISLANDS_CAPITAL, country.Capital);
+        Assert.Equal(ALAND_ISLANDS_NUMERIC_CODE, country.NumericCode);
+        Assert.Equal(ALAND_ISLANDS_ISO2_CODE, country.ISO2Code);
+        Assert.Equal(ALAND_ISLANDS_ISO3_CODE, country.ISO3Code);
+        Assert.Equal(ALAND_ISLANDS_CALLING_CODE, country.CallingCode);
     }
 }

src/World.Net.UnitTests/CountryProviderTest.cs

@@ -4,6 +4,15 @@ namespace World.Net.UnitTests;
 
 public sealed class CountryProviderTest
 {
+    private const string AFGHANISTAN_ISO2_CODE = "AF";
+    private const string AFGHANISTAN_ISO3_CODE = "AFG";
+    private const string AFGHANISTAN_CAPITAL = "Kabul";
+    private const int AFGHANISTAN_NUMERIC_CODE = 004;
+    private const string AFGHANISTAN_CALLING_CODE = "+93";
+    private const int AFGHANISTAN_STATE_COUNT = 34;
+    private const string AFGHANISTAN_NAME = "Afghanistan";
+    private const string AFGHANISTAN_NATIVE_NAME = "افغانستان";
+    private const string AFGHANISTAN_OFFICIAL_NAME = "Islamic Republic of Afghanistan";
     [Fact]
     public void GetAllCountries_ShouldReturn_NonEmptyAndNonNullCollection()
     {
@@ -26,16 +35,16 @@ public void GetCountry_ShouldReturn_Country_WhenCountryExists()
         Assert.NotNull(country);
         Assert.NotEmpty(country.Name);
         Assert.NotEmpty(country.States);
-        Assert.Equal("AF", country.ISO2Code);
-        Assert.Equal("AFG", country.ISO3Code);
-        Assert.Equal("Kabul", country.Capital);
-        Assert.Equal(004, country.NumericCode);
-        Assert.Equal("+93", country.CallingCode);
-        Assert.Equal(34, country.States.Count());
-        Assert.Equal("Afghanistan", country.Name);
+        Assert.Equal(AFGHANISTAN_ISO2_CODE, country.ISO2Code);
+        Assert.Equal(AFGHANISTAN_ISO3_CODE, country.ISO3Code);
+        Assert.Equal(AFGHANISTAN_CAPITAL, country.Capital);
+        Assert.Equal(AFGHANISTAN_NUMERIC_CODE, country.NumericCode);
+        Assert.Equal(AFGHANISTAN_CALLING_CODE, country.CallingCode);
+        Assert.Equal(AFGHANISTAN_STATE_COUNT, country.States.Count());
+        Assert.Equal(AFGHANISTAN_NAME, country.Name);
         Assert.Equal(existingCountryId, country.Id);
-        Assert.Equal("افغانستان", country.NativeName);
-        Assert.Equal("Islamic Republic of Afghanistan", country.OfficialName);        
+        Assert.Equal(AFGHANISTAN_NATIVE_NAME, country.NativeName);
+        Assert.Equal(AFGHANISTAN_OFFICIAL_NAME, country.OfficialName);        
     }
 
     [Fact]

src/World.Net/State.cs

@@ -3,6 +3,9 @@
 
 namespace World.Net;
 
+/// <summary>
+/// Represents a state, province, or region with relevant details such as its name, ISO code, and type.
+/// </summary>
 public class State
 {
     /// <summary>