test: add comprehensive S3 cache resilience tests

Network Failure Tests:
- Add TestS3Cache_NetworkTimeout for temporary vs persistent timeout handling
- Add TestS3Cache_SigstoreOutage for SLSA verification service unavailability
- Add TestS3Cache_ContextCancellation for context cancellation during operations
- Add TestS3Cache_PartialFailure for mixed package success/failure scenarios

Rate Limiting Tests:
- Add TestS3Cache_RateLimiting for S3 rate limit recovery with exponential backoff
- Add TestS3Cache_ConcurrentDownloadsRateLimit for parallel request rate limiting
- Add TestS3Cache_ExponentialBackoff for retry backoff behavior validation
- Add TestS3Cache_MaxRetryLimit for retry exhaustion handling
- Add TestS3Cache_MixedFailureTypes for error categorization and retry logic

Implements configurable failure simulation with realistic error types,
timing simulation, and concurrent access safety. Tests validate graceful
degradation, retry logic, rate limiting, and context handling throughout
the download pipeline.

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

Author: leodido