Add integration tests for Proof Chain and Reachability workflows

- Implement ProofChainTestFixture for PostgreSQL-backed integration tests.
- Create StellaOps.Integration.ProofChain project with necessary dependencies.
- Add ReachabilityIntegrationTests to validate call graph extraction and reachability analysis.
- Introduce ReachabilityTestFixture for managing corpus and fixture paths.
- Establish StellaOps.Integration.Reachability project with required references.
- Develop UnknownsWorkflowTests to cover the unknowns lifecycle: detection, ranking, escalation, and resolution.
- Create StellaOps.Integration.Unknowns project with dependencies for unknowns workflow.

tests/integration/StellaOps.Integration.Reachability/ReachabilityTestFixture.cs

@@ -0,0 +1,91 @@
+// -----------------------------------------------------------------------------
+// ReachabilityTestFixture.cs
+// Sprint: SPRINT_3500_0004_0003_integration_tests_corpus
+// Task: T2 - Reachability Integration Tests
+// Description: Test fixture for reachability integration tests
+// -----------------------------------------------------------------------------
+
+using System.Reflection;
+
+namespace StellaOps.Integration.Reachability;
+
+/// <summary>
+/// Test fixture for reachability integration tests.
+/// Provides access to corpus fixtures and test data.
+/// </summary>
+public sealed class ReachabilityTestFixture
+{
+    private readonly string _corpusBasePath;
+    private readonly string _fixturesBasePath;
+
+    public ReachabilityTestFixture()
+    {
+        var assemblyLocation = Assembly.GetExecutingAssembly().Location;
+        var assemblyDirectory = Path.GetDirectoryName(assemblyLocation)!;
+        
+        _corpusBasePath = Path.Combine(assemblyDirectory, "corpus");
+        _fixturesBasePath = Path.Combine(assemblyDirectory, "fixtures");
+    }
+
+    /// <summary>
+    /// Gets the path to a language-specific corpus directory.
+    /// </summary>
+    /// <param name="language">Language identifier (dotnet, java, python, etc.)</param>
+    /// <returns>Full path to the corpus directory</returns>
+    public string GetCorpusPath(string language)
+    {
+        var corpusPath = Path.Combine(_corpusBasePath, language);
+        
+        if (!Directory.Exists(corpusPath))
+        {
+            throw new DirectoryNotFoundException(
+                $"Corpus directory not found for language '{language}' at: {corpusPath}");
+        }
+        
+        return corpusPath;
+    }
+
+    /// <summary>
+    /// Gets the path to a specific fixture directory.
+    /// </summary>
+    /// <param name="fixtureName">Name of the fixture</param>
+    /// <returns>Full path to the fixture directory</returns>
+    public string GetFixturePath(string fixtureName)
+    {
+        var fixturePath = Path.Combine(_fixturesBasePath, fixtureName);
+        
+        if (!Directory.Exists(fixturePath))
+        {
+            throw new DirectoryNotFoundException(
+                $"Fixture directory not found: {fixturePath}");
+        }
+        
+        return fixturePath;
+    }
+
+    /// <summary>
+    /// Lists all available corpus languages.
+    /// </summary>
+    public IReadOnlyList<string> GetAvailableCorpusLanguages()
+    {
+        if (!Directory.Exists(_corpusBasePath))
+        {
+            return Array.Empty<string>();
+        }
+
+        return Directory.GetDirectories(_corpusBasePath)
+            .Select(Path.GetFileName)
+            .Where(name => !string.IsNullOrEmpty(name))
+            .Cast<string>()
+            .ToList();
+    }
+
+    /// <summary>
+    /// Checks if a corpus exists for the given language.
+    /// </summary>
+    public bool HasCorpus(string language)
+    {
+        var corpusPath = Path.Combine(_corpusBasePath, language);
+        return Directory.Exists(corpusPath);
+    }
+}