Fix: Preserve explicitly nullable attributes

Added explicitlyNullable flag to distinguish between:
- Attributes explicitly marked as nullable in docs ({{<nullable>}}, etc.)
- Attributes marked nullable due to version-based logic

Now removeNullableIfSameVersion only removes version-based nullable flags,
preserving explicitly nullable attributes like Conversation#last_status
and AsyncRefresh#result_count.

Updated tests to verify both cases work correctly.

Co-authored-by: abraham <[email protected]>

Author: Copilot