fix(cron): prevent EBADF crash in cron scripts when parent stdin is closed

[parthchandak02]

What does this PR do?

On macOS, long-running Hermes processes may close fd 0 (stdin) during the tool loop. When subprocess.run launches a child Python process, init_sys_streams calls fstat(0), which fails with EBADF → "Fatal Python error: can't initialize sys standard streams".

This adds stdin=subprocess.DEVNULL to _run_job_script() so fd 0 is always valid in the child. The same pattern is already used in gateway/shutdown_forensics.py and tools/process_registry.py (commit 214b953).

Related Issue

No existing issue — discovered locally on macOS 15.5. Root cause is CPython issue #10806 / loky #420.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• cron/scheduler.py: Added stdin=subprocess.DEVNULL to subprocess.run() in _run_job_script()
• tests/cron/test_cron_script.py: Added test_script_survives_closed_parent_stdin regression test

How to Test

1. Run python -m pytest tests/cron/ -q — all tests pass
2. The regression test closes fd 0 in the parent, then calls _run_job_script and verifies the child runs successfully

Checklist

Code

• My commit message follows Conventional Commits (fix(cron): prevent EBADF crash...)
• I searched for existing PRs to confirm this isn't a duplicate
• My PR contains only changes related to this fix
• I've run python -m pytest tests/cron/ -q and all 389 tests pass
• I've added tests for my changes (regression test included)
• I've tested on my platform: macOS 15.5

Documentation & Housekeeping

• N/A — bug fix, no docs or config changes needed
• N/A — cross-platform: fix is macOS-specific but the stdin=DEVNULL param is harmless on other platforms

Screenshots / Logs

Test output:

tests/cron/test_cron_script.py::TestRunJobScript::test_script_survives_closed_parent_stdin PASSED
389 passed in 7.23s