feat(convert): support explicit type hints in engine value encoding

[lemorage]

Context

#792 (comment)

Cause

The problem was that encode_engine_value couldn't distinguish between:

1. Empty dicts that should be KTables (should become [])
2. Empty dicts that should be JSON objects (should stay {})

Solution

Make encode_engine_value function context-aware by adding an optional _in_struct and type_hint params.

The logic now:
- When encoding struct fields, if the type-hint denotes it is a JSON, it stays what it is, otherwise becomes [] (assuming KTable)
- When encoding standalone values (_in_struct=False): empty dicts stay {}

Pass type hints when calling encode_engine_value function, so it will be explicitly known how we handle each value.

[badmonster0]

Thanks a lot for taking care of this, @lemorage!

[badmonster0]

Thanks a lot for starting this deep refactoring, which solves the problem more systematically!