Add payload logging regression test

[RyanKung]

Summary

• route oversized-payload diagnostics through a metadata-only logging helper
• add a tracing regression test proving custom message body text is not emitted
• keep tx id, message kind, byte count, and limit diagnostics intact

Validation

• cargo +nightly fmt --all --check
• cargo test -p rings-core oversized_payload_log_omits_message_body --features dummy -- --nocapture
• cargo test -p rings-core malformed_outbound_payload_is_rejected_before_connection_admission --features dummy
• git diff --check