Add Day12 implementation with puzzle-solving logic and tests

AoC_2025.Tests/Day12Test.cs

@@ -0,0 +1,47 @@
+using AdvenOfCode.Contracts;
+
+namespace AoC_2025.Tests;
+
+public class Day12Test
+{
+    private IPuzzleSolver<long> _sut;
+    private static readonly string rootPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
+    private readonly string TestInputPath = @$"{rootPath}\AoC-PuzzleInputs\2025\Test\day12.txt";
+    private readonly string ProdInputPath = @$"{rootPath}\AoC-PuzzleInputs\2025\Prod\day12.txt";
+    
+    public Day12Test()
+    {
+        _sut = new Day12();
+    }
+    [Fact]
+    public void Part01_Test_equals_2()
+    {
+        var actual = _sut.SolvePart1(TestInputPath);
+        
+        Assert.Equal(2, actual);
+    }
+    
+    [Fact]
+    public void Part01_Prod_equals_460()
+    {
+        var actual = _sut.SolvePart1(ProdInputPath);
+        
+        Assert.Equal(460, actual);
+    }
+    
+    [Fact]
+    public void Part02_Test2_equals_0()
+    {
+        var actual = _sut.SolvePart2(TestInputPath);
+        
+        Assert.Equal(0, actual);
+    }
+    
+    [Fact]
+    public void Part02_Prod_equals_0()
+    {
+        var actual = _sut.SolvePart2(ProdInputPath);
+        
+        Assert.Equal(0, actual);
+    }
+}