telemetry(exec-server): trace Noise harness relay

[apanasenko-oai]

Summary

Trace the harness side of the encrypted exec-server relay.

• Attribute handshake validation, virtual-stream creation, decrypt, decode, and connection handoff.
• Preserve authorization, failure-budget, and relay-close behavior.
• Remove the temporary traced-message allowance once the harness producer lands.

This PR is stacked on the Noise virtual-stream layer in #30676 and is split out of #30632.

Validation

• The encrypted remote-exec integration test remote_environment_routes_encrypted_exec_server_rpc passed.
• just fmt passed, with an unrelated justfile rewrite discarded.
• The broader exec-server suite remains environment-limited by macOS sandbox-exec permission denials and local network or HTTP timeouts.