* Fix: preserve request body for STS signature verification
- Save and restore request body in UnifiedPostHandler after ParseForm()
- This allows STS handler to verify signatures correctly
- Fixes 'invalid AWS signature: 53' error (ErrContentSHA256Mismatch)
- ParseForm() consumes the body, so we need to restore it for downstream handlers
* Improve error handling in UnifiedPostHandler
- Add http.MaxBytesReader to limit body size to 10 MiB (iamRequestBodyLimit)
- Add proper error handling for io.ReadAll failures
- Log errors when body reading fails
- Prevents DoS attacks from oversized request bodies
- Addresses code review feedback