telemetry(exec-server): trace RPC transport

[apanasenko-oai]

Summary

Propagate one trace across exec-server RPC and processor dispatch.

• Carry W3C context through request and notification envelopes without changing the wire format.
• Measure queue residence, serialization, write, decode, response dispatch, and processor execution.
• Correlate terminal close receipt and publication with the originating process trace.
• Add focused RPC and context-propagation tests.

This is the exec transport foundation split out of #30632.

Validation

• Seven focused RPC, processor, connection, disconnect, and trace-context 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.