fix complex type repr

[jackyzha0]

Why

  File "/Users/jzhao/projects/ai-infra/.venv/lib/python3.12/site-packages/replit_river/codegen/client.py", line 408, in encode_type
    raise ValueError(f"What does it mean to have {_o2} here?")
                                                 ^^^^^
  File "/Users/jzhao/projects/ai-infra/.venv/lib/python3.12/site-packages/replit_river/codegen/typing.py", line 33, in __str__
    raise Exception("Complex type must be put through render_type_expr!")
Exception: Complex type must be put through render_type_expr!

What changed

render the error correctly

Test plan

codegen doesnt crash

[jackyzha0]

theres a deeper problem here with nested datetypes in a recursive type lol, im just going to not codegen the offending proc for now