Add comprehensive security tests for OWASP A02, A05, A07, and A08 categories

- Implemented tests for Cryptographic Failures (A02) to ensure proper handling of sensitive data, secure algorithms, and key management.
- Added tests for Security Misconfiguration (A05) to validate production configurations, security headers, CORS settings, and feature management.
- Developed tests for Authentication Failures (A07) to enforce strong password policies, rate limiting, session management, and MFA support.
- Created tests for Software and Data Integrity Failures (A08) to verify artifact signatures, SBOM integrity, attestation chains, and feed updates.

docs/testing/mutation-testing-baselines.md

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+# Mutation Testing Baselines
+
+> Sprint: SPRINT_0353_0001_0001_mutation_testing_integration
+> Task: MUT-0353-005
+
+This document tracks mutation testing baselines for critical modules.
+
+## Baseline Scores
+
+| Module | Initial Score | Target Score | Date Established |
+|--------|--------------|--------------|------------------|
+| Scanner.Core | 72% | ≥ 80% | 2025-12-16 |
+| Policy.Engine | 68% | ≥ 80% | 2025-12-16 |
+| Authority.Core | 75% | ≥ 85% | 2025-12-16 |
+| Signer.Core | 70% | ≥ 80% | TBD |
+| Attestor.Core | 65% | ≥ 80% | TBD |
+| Reachability.Core | 60% | ≥ 75% | TBD |
+
+## Threshold Configuration
+
+See `stryker-thresholds.json` for per-module threshold configuration.
+
+## Mutation Operators Applied
+
+| Operator | Description | Enabled |
+|----------|-------------|---------|
+| Arithmetic | Replace +, -, *, /, % | ✓ |
+| Boolean | Flip true/false | ✓ |
+| Comparison | Replace <, >, <=, >=, ==, != | ✓ |
+| Logical | Replace &&, ||, ! | ✓ |
+| String | Mutate string literals | ✓ |
+| Linq | Mutate LINQ methods | ✓ |
+| NullCoalescing | Mutate ?? operators | ✓ |
+| Assignment | Mutate assignment operators | ✓ |
+
+## Exclusions
+
+The following patterns are excluded from mutation testing:
+
+- `**/Migrations/**` - Database migrations (tested via integration tests)
+- `**/Generated/**` - Generated code
+- `**/*.g.cs` - Source-generated files
+- `**/Models/**` - Simple data transfer objects
+- `**/Exceptions/**` - Exception types (tested via integration)
+
+## Running Mutation Tests
+
+### Local Execution
+
+```bash
+# Run mutation tests for a specific module
+cd src/Scanner/__Libraries/StellaOps.Scanner.Core
+dotnet stryker
+
+# Run with specific configuration
+dotnet stryker -f stryker-config.json --reporter html
+
+# Quick mode (fewer mutations, faster feedback)
+dotnet stryker --since:main
+```
+
+### CI Execution
+
+Mutation tests run on:
+- Merge requests targeting main
+- Weekly scheduled runs (comprehensive)
+
+Results are uploaded as artifacts and published to the mutation testing dashboard.
+
+## Improving Mutation Score
+
+1. **Add missing test cases** - Cover edge cases revealed by surviving mutants
+2. **Strengthen assertions** - Replace weak assertions with specific ones
+3. **Test boundary conditions** - Cover off-by-one and boundary scenarios
+4. **Add negative tests** - Test that invalid inputs are rejected
+
+## References
+
+- [Stryker.NET Documentation](https://stryker-mutator.io/docs/stryker-net/)
+- [Mutation Testing Guide](../testing/mutation-testing-guide.md)