* Fix: Populate Claims from STS session RequestContext for policy variable substitution
When using STS temporary credentials (from AssumeRoleWithWebIdentity) with
AWS Signature V4 authentication, JWT claims like preferred_username were
not available for bucket policy variable substitution (e.g., ${jwt:preferred_username}).
Root Cause:
- STS session tokens store user claims in the req_ctx field (added in PR #8079)
- validateSTSSessionToken() created Identity but didn't populate Claims field
- authorizeWithIAM() created IAMIdentity but didn't copy Claims
- Policy engine couldn't resolve ${jwt:*} variables without claims
Changes:
1. auth_signature_v4.go: Extract claims from sessionInfo.RequestContext
and populate Identity.Claims in validateSTSSessionToken()
2. auth_credentials.go: Copy Claims when creating IAMIdentity in
authorizeWithIAM()
3. auth_sts_identity_test.go: Add TestSTSIdentityClaimsPopulation to
verify claims are properly populated from RequestContext
This enables bucket policies with JWT claim variables to work correctly
with STS temporary credentials obtained via AssumeRoleWithWebIdentity.
Fixes#8037
* Refactor: Idiomatic map population for STS claims
5472061231