Add malformed-output tests to the custom adapter starter

[myheart521]

Goal

Make the validation boundary in examples/custom_adapter explicit through focused tests.

Acceptance criteria

• Add a parametrized test for NaN and positive/negative infinity coordinates.
• Add a test for a coordinate tuple with the wrong dimension.
• Add a test for a non-positive requested horizon.
• Assert concise ValueError messages without overfitting the full sentence.
• Keep the example dependency-free and do not change valid output artifacts.
• Run ruff, mypy, and the custom-adapter test module.

Starting point

See tests/test_custom_adapter_example.py and examples/custom_adapter/adapter.py after the contributor-readiness PR is merged.