[miki3421]

A malformed cron annotation in one namespace could raise an uncaught exception and stop the shared scheduler before it processed other tenants. Validate the annotation as a non-empty string of at most 4096 characters before parsing, and isolate processing errors to the individual action so the remaining batch continues.

The regression tests cover non-string/oversized values, a malformed action followed by another tenant's valid recurring and one-shot actions, namespace credential selection, and one-shot unscheduling. The existing recurring/autoexec behavior is preserved for valid input.

Fixes apache/openserverless#275.

Validation:

• Focused unittest suite passed inside the baseline image with pinned croniter 1.4.1 and networking disabled.
• Existing actionexecutor doctests and Python syntax checks passed; git diff --check is clean.
• Live K3s 0.9.0 lab: Alpha and Beta each produced two fresh scheduled heartbeats while a malformed Alpha action existed. A job logged the malformed annotation before successfully dispatching Beta's heartbeat; the malformed action was never invoked. Alpha/Beta source was unchanged. The probe was removed and the original CronJob image was restored afterward.