test: add comprehensive SLSA attestation generation tests

- Add TestGenerateSLSAAttestation_Format for JSON structure validation
- Add TestGenerateSLSAAttestation_RequiredFields for mandatory field checks
- Add TestGenerateSLSAAttestation_PredicateContent for predicate validation
- Add TestGenerateSLSAAttestation_ChecksumAccuracy with multiple content types
- Add TestGenerateSLSAAttestation_ChecksumConsistency for deterministic hashing
- Add TestGenerateSLSAAttestation_GitHubContextIntegration for CI/CD scenarios
- Add TestGenerateSLSAAttestation_InvalidGitHubContext for error handling
- Add TestGenerateSLSAAttestation_FileErrors for file system edge cases
- Add TestComputeSHA256_EdgeCases for hash computation validation
- Add TestGitHubContext_Validation for context structure validation
- Add TestGenerateSignedSLSAAttestation_Integration for end-to-end testing
- Add TestSignedAttestationResult_Structure for result format validation
- Add TestGetGitHubContext for environment variable extraction
- Add TestSigningError for error type validation and categorization
- Add TestWithRetry for retry logic validation with exponential backoff
- Add TestCategorizeError for error classification testing

Provides comprehensive coverage of SLSA attestation generation, validation,
error handling, and retry mechanisms with 63.0% code coverage.

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido