Fix: Avoid InvalidCastException in TickerQHubNotificationSender occurrence serialization

[mdemrulkayes]

Related Issue

Fixes #863

AddCronOccurrenceAsync and UpdateCronOccurrenceAsync previously hard-cast object occurrence to CronTickerOccurrenceEntity<CronTickerEntity>, throwing InvalidCastException
when a different concrete type was passed

• Extracted SerializeOccurrence helper that attempts a safe is cast first; falls back to generic JsonSerializer.SerializeToElement with camelCase policy
• Added static CamelCaseOptions field to avoid allocating JsonSerializerOptions on every fallback call

Root cause

Direct cast (CronTickerOccurrenceEntity<CronTickerEntity>)occurrence fails at runtime when occurrence is a subtype or unrelated type that doesn't satisfy the cast.

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[mdemrulkayes]

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