fix(engine): inherit background mode in subworkflows

[Artur Leao (arturleao)]

Summary

• Propagate background-mode context from parent workflows into child workflow engines.
• Prevent nested human_gate steps from racing a CLI stdin prompt under --web-bg.
• Add regression coverage for a dashboard-backed nested gate with TTY-like stdin.

Why

Only the CLI builds RunContext. Child engines previously saw bg_mode=False, so a nested gate could call Prompt.ask, raise EOFError, and fail a background workflow even while a web dashboard was available.

Testing

• tests/test_engine/test_subworkflow.py (42 passed)
• Scoped Ruff and formatting checks

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   92.00%           
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  Files           ?      165           
  Lines           ?    27335           
  Branches        ?        0           
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  Hits            ?    25149           
  Misses          ?     2186           
  Partials        ?        0           

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[Jason Robert (jrob5756)]

LGTM, thanks for contributing!