Avoid formatting complete MessagePayload values in transport error logs

[RyanKung]

Problem

crates/core/src/swarm/transport.rs logs the complete MessagePayload when rejecting an oversized message:

tracing::error!("Message is too large: {:?}", payload);

Payloads can contain application data, transaction and relay metadata. Logging the complete structure is unnecessarily risky for deployments that treat transport/application contents as sensitive.

Proposed direction

Log only diagnostic metadata required to investigate the failure: encoded size, configured limit, transaction ID (if appropriate), message kind, and non-sensitive routing identifiers. Do not format message bodies, ciphertext, or arbitrary custom-message bytes.

Acceptance criteria

• The oversized-message log path does not serialize or debug-format the full payload.
• The replacement log retains size/limit and a safe correlation identifier.
• Add a test or logging-focused regression check that sensitive custom-message content is absent from the emitted diagnostic.