fix: stabilize RSK SIWE address assertion

[DenisCarriere]

Summary

• compare the SIWE address case-insensitively in the account authentication test
• preserve validation that the generated message contains the requested address
• avoid treating RSKIP-60 casing and the SIWE serializer's Ethereum checksum casing as different addresses

Why

The expanded master matrix repeatedly failed the RSK web test when a generated address had different valid checksum casing under RSKIP-60 and the SIWE serializer. The assertion was case-sensitive even though the address bytes were identical.

Validation

• formatted the changed test with Elixir 1.19.4 / OTP 27 in a container
• git diff --check
• CodeQL: passed
• default GitHub Actions matrix: passed
• ci:rsk GitHub Actions matrix: passed
• RSK Blockscout Web suite: 29 doctests and 1,688 tests, 0 failures