fix: restore outer RefFactory after nested JSON deserialization

[zantvoort]

The JSON converters in storm-jackson2, storm-jackson3, and storm-kotlinx-serialization bound the deserialization RefFactory with set and finally remove. A nested fromDatabase call, reachable through a custom deserializer that issues a query, removed the outer factory on its way out. The outer record's remaining Ref fields then saw no factory and fell back to detached refs, with no exception or log.

Each converter now saves the previous ThreadLocal value before binding and restores it on exit, removing the entry only when there was no previous value. This is the same save-and-restore discipline TransactionScope uses in core.

Each module gains a regression test in which a payload's first field carries a custom deserializer that runs a nested query (mapping Owner, whose @Json address field re-enters fromDatabase), followed by a Ref field. The test asserts the ref is attached and fetches. All three tests fail against the previous code with isFetchable() returning false.

Note on the kotlinx test: a @Contextual Ref<T> property inside a @Serializable class requires T itself to be @Serializable, since the compiler plugin resolves type-argument serializers even for contextual properties. The test therefore uses a @Serializable entity mapped to the owner table as the ref target.

Fixes #385

[codecov]

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