fix(workflows): surface dry-run previews on engine exception

When the engine raises mid-run (e.g. template resolution failure),
the dry-run previews resolved by earlier steps are the most useful
debug signal a user can get. They were being silently dropped because
the CLI's exception path bypassed the preview loop.

This change:
* Engine: attach the partially-populated state to the raised
  exception as exc.partial_state (mirrors in both execute and resume).
* CLI: extract the preview loop into _print_dry_run_previews(state)
  and call it from both exception handlers when --dry-run is set and
  --json is not. The --json contract (stdout is a single JSON object)
  is preserved by skipping the preview print in that branch.
* Tests: add test_run_dry_run_prints_previews_on_engine_exception
  and test_run_dry_run_no_previews_when_json covering the success
  and --json-failure paths.

Addresses review comment 3423394535 on PR #2704.

Author: fuleinist