Summary

• Include captured stderr in ExecError display output when stderr is non-empty.
• Keep empty stderr out of the rendered message to avoid noisy (stderr: ) suffixes.
• Add focused unit coverage for both display behaviors.

Why

PlatformNetwork/bounty-challenge#50304 reports that ExecError::fmt silently omits the stderr field even though failed command stderr is captured on the error object. This makes displayed non-zero-exit errors much less actionable.

Verification

• git diff --check HEAD~1..HEAD
• Static source check confirmed the stderr render branch, empty-stderr guard, and targeted tests are present.

Note: this WSL cron environment does not have cargo/rustc, so I could not run the Rust test suite locally here. The new tests are included for CI/local Rust verification.

Closes PlatformNetwork/bounty-challenge#50304