Implement Option B (enhanced env var) for T058.S2:
- Support multiple S3 credentials via S3_CREDENTIALS env var
- Format: "key1:secret1,key2:secret2,..."
- Backward compatible with S3_ACCESS_KEY_ID/S3_SECRET_KEY
- Add tests for both multi and single credential formats
This unblocks T039 production deployment while proper IAM
credential service (T060) is implemented separately.
Tests: 10/10 auth tests pass (added 2 new credential tests)
Refs: T058.S2 Option B (approved), T060 (proper IAM integration)
- Replace form_urlencoded with RFC 3986 compliant URI encoding
- Implement aws_uri_encode() matching AWS SigV4 spec exactly
- Unreserved chars (A-Z,a-z,0-9,-,_,.,~) not encoded
- All other chars percent-encoded with uppercase hex
- Preserve slashes in paths, encode in query params
- Normalize empty paths to '/' per AWS spec
- Fix test expectations (body hash, HMAC values)
- Add comprehensive SigV4 signature determinism test
This fixes the canonicalization mismatch that caused signature
validation failures in T047. Auth can now be enabled for production.
Refs: T058.S1
