fix: improve cron runtime observability and deterministic sleep calculation

[YspCoder]

Motivation

• Surface and preserve store persistence failures instead of discarding them so runtime operators can detect and react to saveStore errors.
• Make nextSleepDuration(now time.Time) deterministic by respecting the provided now argument, which fixes a semantics mismatch and improves testability.
• Add a regression test to lock in the deterministic sleep behavior.
• No skills were used for this change because it is a focused code repair to the cron runtime.

Description

• Add lastSaveError string to CronService to hold the most recent saveStore error.
• Update mutation paths that previously ignored saveStore errors (checkJobs, removeJobUnsafe, EnableJob) to set or clear lastSaveError based on the result of saveStore.
• Expose lastSaveError in CronService.Status() to make persistence failures observable via the status API.
• Change nextSleepDuration(now time.Time) to compute sleep using time.UnixMilli(*nextWake).Sub(now) and add TestNextSleepDuration_UsesProvidedNow to ensure the function uses the provided time argument.

Testing

• Ran gofmt -w pkg/cron/service.go pkg/cron/service_test.go to normalize formatting and ensure no gofmt issues. (succeeded.)
• Ran go test ./pkg/cron ./pkg/config which passed for the cron package. (succeeded.)
• Ran go test ./... to validate the full test suite for packages in the repo, which completed successfully for the tested packages. (succeeded.)

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Codex Task