Add unit tests for codegen

The initial codegen is missing some things and does not yet produce valid code, since it was primarily targeting generation of constraints.

The following should be added:
- any missing required codegen in order for output files to be valid
- Unit tests for each of the generated methods / structs

Look at the existing codegen implementation to understand what methods and structs are generated. Identify missing pieces needed for valid output. Write unit tests for each generated method/struct, likely in the test directory. Run existing tests to ensure new tests integrate correctly.

索引モデルが issue の本文から書いたものです。