telemetry(exec-server): trace Noise virtual streams

[apanasenko-oai]

Summary

Carry RPC trace context through exec-server Noise virtual streams and physical relay frames.

• Reuse one message trace across fragmentation and reassembly.
• Measure stream queueing, framing, encryption, physical send, receive, decode, and processor handoff.
• Correlate responses and terminal notifications with their originating request.
• Preserve relay framing and virtual-stream behavior.

This PR is stacked on the exec RPC foundation in #30675 and is split out of #30632.

Validation

• Nine focused Noise stream, trace-correlation, keepalive, and malformed-handshake tests 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.