* Fix STS AssumeRole with POST body param and add integration test
* Add STS integration test to CI workflow
* Address code review feedback: fix HPP vulnerability and style issues
* Refactor: address code review feedback
- Fix HTTP Parameter Pollution vulnerability in UnifiedPostHandler
- Refactor permission check logic for better readability
- Extract test helpers to testutil/docker.go to reduce duplication
- Clean up imports and simplify context setting
* Add SigV4-style test variant for AssumeRole POST body routing
- Added ActionInBodyWithSigV4Style test case to validate real-world scenario
- Test confirms routing works correctly for AWS SigV4-signed requests
- Addresses code review feedback about testing with SigV4 signatures
* Fix: always set identity in context when non-nil
- Ensure UnifiedPostHandler always calls SetIdentityInContext when identity is non-nil
- Only call SetIdentityNameInContext when identity.Name is non-empty
- This ensures downstream handlers (embeddedIam.DoActions) always have access to identity
- Addresses potential issue where empty identity.Name would skip context setting
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