test(s3): expand service and dashboard coverage

[simota]

Summary

• Add targeted S3 service edge/error tests for bucket, object, metadata, copy, multipart, and presigned URL behavior.
• Add dashboard /api/s3 coverage for validation, disabled state, metadata propagation, mutation safety, and destructive paths.
• Add S3 test coverage autoloop with service, dashboard API, E2E/docs, and coverage threshold gates.

Verification

• VERIFY_STAGE=full-test-coverage bash scripts/s3-test-coverage-autoloop/verify.sh
  • passed=7 failed=0
• go test -cover ./internal/services/s3
  • coverage: 72.6% of statements
• go test -cover ./internal/dashboard
  • coverage: 69.8% of statements

Notes

• Test-focused change. Product behavior should remain unchanged except for defects a new test would expose.
• Tests avoid external AWS calls and must not log credentials, Authorization headers, SigV4 material, presigned secrets, or object payloads.