test: Add comprehensive test suite for Day 2 solution

test/advent_code2024/solutions/day02/day2_test.exs

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+defmodule AdventCode2024.Solutions.Day2Test do
+  use ExUnit.Case
+  alias AdventCode2024.Solutions.Day2
+
+  describe "solve/1" do
+    test "correctly identifies safe reports from example data" do
+      input = """
+      7 6 4 2 1
+      1 2 7 8 9
+      9 7 6 2 1
+      1 3 2 4 5
+      8 6 4 4 1
+      1 3 6 7 9
+      """
+
+      assert Day2.solve(input) == {:ok, 2}
+    end
+
+    test "identifies a single safe decreasing report" do
+      input = "7 6 4 2 1\n"
+      assert Day2.solve(input) == {:ok, 1}
+    end
+
+    test "identifies a single safe increasing report" do
+      input = "1 3 6 7 9\n"
+      assert Day2.solve(input) == {:ok, 1}
+    end
+
+    test "identifies an unsafe report with too large increase" do
+      input = "1 2 7 8 9\n"
+      assert Day2.solve(input) == {:ok, 0}
+    end
+
+    test "identifies an unsafe report with too large decrease" do
+      input = "9 7 6 2 1\n"
+      assert Day2.solve(input) == {:ok, 0}
+    end
+
+    test "identifies an unsafe report with direction change" do
+      input = "1 3 2 4 5\n"
+      assert Day2.solve(input) == {:ok, 0}
+    end
+
+    test "identifies an unsafe report with no change between numbers" do
+      input = "8 6 4 4 1\n"
+      assert Day2.solve(input) == {:ok, 0}
+    end
+
+    test "handles empty input" do
+      assert Day2.solve("") == {:error, :no_valid_reports}
+    end
+
+    test "handles invalid input format" do
+      assert Day2.solve("not numbers") == {:error, :invalid_format}
+    end
+  end
+end