Add tests for SBOM generation determinism across multiple formats

- Created `StellaOps.TestKit.Tests` project for unit tests related to determinism.
- Implemented `DeterminismManifestTests` to validate deterministic output for canonical bytes and strings, file read/write operations, and error handling for invalid schema versions.
- Added `SbomDeterminismTests` to ensure identical inputs produce consistent SBOMs across SPDX 3.0.1 and CycloneDX 1.6/1.7 formats, including parallel execution tests.
- Updated project references in `StellaOps.Integration.Determinism` to include the new determinism testing library.

src/__Libraries/__Tests/StellaOps.Cryptography.Tests/Sha256DigestTests.cs

@@ -0,0 +1,52 @@
+using System.Security.Cryptography;
+using System.Text;
+using StellaOps.Cryptography;
+using StellaOps.Cryptography.Digests;
+using Xunit;
+
+namespace StellaOps.Cryptography.Tests;
+
+public sealed class Sha256DigestTests
+{
+    [Fact]
+    public void Normalize_AllowsBareHex_WhenPrefixNotRequired()
+    {
+        var hex = new string('a', Sha256Digest.HexLength);
+        Assert.Equal($"sha256:{hex}", Sha256Digest.Normalize(hex));
+    }
+
+    [Fact]
+    public void Normalize_NormalizesPrefixAndHexToLower()
+    {
+        var hexUpper = new string('A', Sha256Digest.HexLength);
+        Assert.Equal(
+            $"sha256:{new string('a', Sha256Digest.HexLength)}",
+            Sha256Digest.Normalize($"SHA256:{hexUpper}"));
+    }
+
+    [Fact]
+    public void Normalize_RequiresPrefix_WhenConfigured()
+    {
+        var hex = new string('a', Sha256Digest.HexLength);
+        var ex = Assert.Throws<FormatException>(() => Sha256Digest.Normalize(hex, requirePrefix: true, parameterName: "sbomDigest"));
+        Assert.Contains("sbomDigest", ex.Message, StringComparison.Ordinal);
+        Assert.Contains("sha256:", ex.Message, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void ExtractHex_ReturnsLowercaseHex()
+    {
+        var hexUpper = new string('A', Sha256Digest.HexLength);
+        Assert.Equal(new string('a', Sha256Digest.HexLength), Sha256Digest.ExtractHex($"sha256:{hexUpper}"));
+    }
+
+    [Fact]
+    public void Compute_UsesCryptoHashStack()
+    {
+        var hash = CryptoHashFactory.CreateDefault();
+        var content = Encoding.UTF8.GetBytes("hello");
+
+        var expectedHex = Convert.ToHexStringLower(SHA256.HashData(content));
+        Assert.Equal($"sha256:{expectedHex}", Sha256Digest.Compute(hash, content));
+    }
+}