Fix compiler warnings and errors

[captkirk88]

This PR fixes several compiler warnings and errors when building with recent versions of the Beef compiler:

1. Resolves duplicate variable declarations and scope conflicts (BF4200/BF0168).
2. Fixes a compiler nullable overload/specialization issue by renaming target-based Deserialize overloads to DeserializeInto and using 'default' instead of 'null' for Nullable struct resets.
3. Adds DistinctOptions to Atma.Json.Tests to preserve reflection metadata for generic lists and dictionaries instantiated in tests.