telemetry(exec-server): trace remote client lifecycle

[apanasenko-oai]

Summary

Trace remote exec-server connection and client lifecycle stages.

• Attribute initial connection, reconnect, process start, read, and result waits.
• Record cached, initial, and reconnect paths without changing recovery behavior.
• Add spans around environment registration, backend start, and remote reads.

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

Validation

• Eleven focused connection, reconnect, environment, registration, and process-start 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.