Make orjson optional for JSON handling

[FranciscoMaxwell]

Closes #423.

Summary

• routes Kaleido JSON serialization/deserialization through a small backend wrapper
• keeps using orjson when it is installed, but falls back to the stdlib json module when it is unavailable
• preserves handling for Decimal, datetime-like values, and list-like/numpy-style values
• removes orjson from required project dependencies and updates fallback coverage

Validation

• python -m compileall kaleido\_json.py kaleido\_kaleido_tab\_tab.py kaleido\mocker\_utils.py tests\test_orjson_encoder.py tests\test_kaleido_tab.py
• direct fallback smoke test with orjson set to None
• git diff --check

I could not run the full pytest/ruff/uv validation locally because this environment does not have pytest, ruff, uv, or Kaleido runtime dependencies such as choreographer installed.

[camdecoster]

Thanks for the PR! This change is already covered in #454, which itself may be rendered moot by some upcoming changes when I upgrade choreographer in this repo. I'm closing this PR in favor of the existing one. Feel free to comment over there.