selfdrived: surface processNotRunning alert when not engaged

[ChenSiyun1234]

Fixes #34751.

When a process crashes, processNotRunning is raised correctly, but it only had NO_ENTRY and SOFT_DISABLE alerts, no PERMANENT one. So when you're not engaged, the screen shows the PERMANENT alert of some downstream fault instead (e.g. steerUnavailable's "LKAS Fault: Restart the car to engage"), which hides the real reason and sends you down the wrong path when debugging. That's exactly what happened in the issue.

This adds a PERMANENT alert for processNotRunning, at Priority.LOW on purpose:

• above the downstream fault's permanent alert (LOWER), so the real cause shows when not engaged, but
• below the soft-disable "TAKE CONTROL IMMEDIATELY" alert (MID), so it never hides the take-control warning if a process dies while engaged.

Added two regression tests in test_alerts.py: one checks processNotRunning wins over steerUnavailable when not engaged, the other checks it does NOT mask the soft-disable alert when engaged. pytest selfdrive/selfdrived/tests/test_alerts.py passes.

[github-actions]

Process replay diff report

Replays driving segments through this PR and compares the behavior to master.
Please review any changes carefully to ensure they are expected.

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