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/AirGap/__Tests/StellaOps.AirGap.Importer.Tests/Reconciliation/ArtifactIndexDigestNormalizationTests.cs

@@ -0,0 +1,43 @@
+using StellaOps.AirGap.Importer.Reconciliation;
+
+namespace StellaOps.AirGap.Importer.Tests.Reconciliation;
+
+public sealed class ArtifactIndexDigestNormalizationTests
+{
+    [Fact]
+    public void NormalizeDigest_AcceptsBareHex()
+    {
+        var digest = new string('A', 64);
+
+        var normalized = ArtifactIndex.NormalizeDigest(digest);
+
+        Assert.Equal("sha256:" + new string('a', 64), normalized);
+    }
+
+    [Fact]
+    public void NormalizeDigest_AcceptsPrefixedSha256()
+    {
+        var digest = "SHA256:" + new string('F', 64);
+
+        var normalized = ArtifactIndex.NormalizeDigest(digest);
+
+        Assert.Equal("sha256:" + new string('f', 64), normalized);
+    }
+
+    [Fact]
+    public void NormalizeDigest_RejectsUnsupportedAlgorithm()
+    {
+        var digest = "sha512:" + new string('a', 128);
+
+        Assert.Throws<FormatException>(() => ArtifactIndex.NormalizeDigest(digest));
+    }
+
+    [Fact]
+    public void NormalizeDigest_RejectsNonHex()
+    {
+        var digest = "sha256:" + new string('g', 64);
+
+        Assert.Throws<FormatException>(() => ArtifactIndex.NormalizeDigest(digest));
+    }
+}
+