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.ProofChain/StellaOps.Integration.ProofChain.csproj

@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- 
+  StellaOps.Integration.ProofChain.csproj
+  Sprint: SPRINT_3500_0004_0003_integration_tests_corpus
+  Task: T1 - Proof Chain Integration Tests
+  Description: End-to-end integration tests for proof chain workflow
+-->
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
+    <PackageReference Include="xunit" Version="2.7.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.8">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="FluentAssertions" Version="6.12.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.0" />
+    <PackageReference Include="Testcontainers" Version="3.6.0" />
+    <PackageReference Include="Testcontainers.PostgreSql" Version="3.6.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <!-- Scanner WebService for integration testing -->
+    <ProjectReference Include="../../../src/Scanner/StellaOps.Scanner.WebService/StellaOps.Scanner.WebService.csproj" />
+    
+    <!-- Proof chain and attestation libraries -->
+    <ProjectReference Include="../../../src/Attestor/__Libraries/StellaOps.Attestor.ProofChain/StellaOps.Attestor.ProofChain.csproj" />
+    <ProjectReference Include="../../../src/Attestor/__Libraries/StellaOps.Attestor.Dsse/StellaOps.Attestor.Dsse.csproj" />
+    
+    <!-- Policy scoring -->
+    <ProjectReference Include="../../../src/Policy/__Libraries/StellaOps.Policy.Scoring/StellaOps.Policy.Scoring.csproj" />
+    
+    <!-- Cryptography -->
+    <ProjectReference Include="../../../src/__Libraries/StellaOps.Cryptography/StellaOps.Cryptography.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="../../fixtures/**/*">
+      <Link>fixtures/%(RecursiveDir)%(Filename)%(Extension)</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
+</Project>