internal(defers): improve defer validation (#4649)

## Description

- Before persisting an event's session meta, let's ensure we aren't
stuffing in too much data
- With other session entry points, we do our resolved validation before
persisting, which would catch wildly large sessions. With defers
however, we persist once before resolving and then persisting again as
the triggering event. We add this new validation to protect our first
persistence.
- We also check for a valid JSON shape before persistence
- EXE-2035
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## Release note

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notes. -->
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## Migration note

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>