Add comprehensive integrity verification tests
- Round-trip verification: upload → download → verify hash matches
- Client-side verification workflow: pre-upload hash computation, checksum header validation
- Size variant tests: integrity verification for 1KB, 100KB, 1MB, 10MB files
- Consistency check endpoint: response format, after upload, limit parameter
- Digest header tests: RFC 3230 format validation, base64 encoding
- Verification modes: pre-verification and streaming verification
- Corruption detection tests:
- Single bit flip detection
- Truncated content detection
- Appended content detection
- Client-side hash mismatch detection
- Consistency check detects size mismatches
- Consistency check detects missing S3 objects
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