Add regression tests for commands diagnostics and verbose tracing

## Summary
Add CI coverage to prevent regressions in commands-step error visibility behavior.

## Scope
- Add workflow test that intentionally fails inside `inputs.commands` and verifies actionable `::error` annotation behavior
- Add workflow test path for `verbose: true` to verify trace output is emitted for command execution and internal output mapping
- Keep tests focused on diagnostics contract (not business logic)

## Why
Recent diagnostics improvements are high-value for downstream users; tests should lock this in.