Fix list schedules context desez (#694)

If a schedule's context stores data that refers to objects that
disappeared, deserialize fail. This produces a poor UX for the list
schedules path, which is entirely denied if a single schedule context is
rotten:
<img width="782" height="574" alt="Screenshot 2026-05-29 at 17 51 02"
src="https://github.com/user-attachments/assets/344aa3b4-d840-4e7e-9693-3813c704bc63"
/>

Reproducible with the test:

<img width="844" height="417" alt="Screenshot 2026-05-29 at 18 06 33"
src="https://github.com/user-attachments/assets/32deee48-6a87-4ea3-beac-0281aabb48cb"
/>

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Co-authored-by: Peter Kraft <petereliaskraft@gmail.com>