Add boundary tests for public protocol limits

[rufatixx]

Goal

Add focused tests for the public limits in RelayProtocol and the validation paths in RelayCryptography and RelayPacketValidator.

Cases

• payload exactly at 256 KiB and one byte above
• node ID exactly at the maximum length and one character above
• content type exactly at the maximum length and one character above
• TTL at zero, at the seven-day maximum, and above the maximum
• empty required identifiers

Acceptance criteria

• Tests are deterministic and do not depend on wall-clock time.
• Both accepted boundaries and rejected values are covered.
• Existing public behavior is preserved.
• dotnet test tests/RelayOS.Core.Tests passes.

Please keep the pull request test-only unless a test reveals an actual bug.

[slegarraga]

Happy to take this one. I will add deterministic boundary tests for the public protocol limits and validation paths, keeping the PR test-only.