* fix(s3): apply PutObject multipart expansion to STS session policy evaluation (#8929)
PR #8445 added logic to implicitly grant multipart upload actions when
s3:PutObject is authorized, but only in the S3 API policy engine's
CompiledStatement.MatchesAction(). STS session policies are evaluated
through the IAM policy engine's matchesActions() -> awsIAMMatch() path,
which did plain pattern matching without the multipart expansion.
Add the same multipart expansion logic to the IAM policy engine's
matchesActions() so that session policies containing s3:PutObject
correctly allow multipart upload operations.
* fix: make multipart action set lookup case-insensitive and optimize
Address PR review feedback:
- Lowercase multipartActionSet keys and use strings.ToLower for lookup,
since AWS IAM actions are case-insensitive
- Only check for s3:PutObject permission when the requested action is
actually a multipart action, avoiding unnecessary awsIAMMatch calls
- Add test case for case-insensitive multipart action matching
b8fc99a9cd