internal(defers): store defer meta in its own hash (#4705)

## Description

- Previously, we added the events meta into the general meta field in
Redis. However this exposed it to lossy cjson serialization
- We split it into a separate field in the Redis hash. Unfortunately,
defer-meta is taken, so instead we use defer-control-meta as our key.
This separate field doesn't need that serialization, and thus stays
pristine

## Release note

<!-- User-facing release note. Use "None" if this does not need release
notes. -->
- Add in support for propagating sessions into child runs, building on
top of previous PRs

## Migration note

<!-- Upgrade, deployment, config, schema, or compatibility notes. Use
"None" if none. -->
None.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>