modelJobToSubmit built the C job_startinfo_t without setting execuser, so the
controller forwarded a NULL execuser in the startjob request and the worker
ran every job as "nobody". Now the submit carries the resolved exec user: an
explicit exec_user wins, otherwise the job runs as the submitting user.
- add resolveExecUser() (plain-Go, unit-testable) + TestResolveExecUser
- set si.execuser in modelJobToSubmit, free it in freeJobSubmit
Note: deploying this exposed an ASAN heap-use-after-free in the apiserver's
submit/notification path (strncpy reading freed memory); the fix is committed
but needs the memory-lifecycle investigation before it can ship safely.
Co-Authored-By: Claude <noreply@anthropic.com>
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