TODO: improve default for shape handling

## Description

Replace fallback TODO with explicit behavior
See [src/utils/wellbores/data/details/index.ts](https://github.com/equinor/videx-map/blob/be54d2838b2a96bb5610b0b7cf6f50a6cb002068/src/utils/wellbores/data/details/index.ts#L23)

## Acceptance criteria

1. Fix: throw descriptive error or add explicit default shape contract

Start at src/utils/wellbores/data/details/index.ts line 23 and trace the shape fallback through its callers to understand the expected contract. Decide whether the fallback should be rejected with a descriptive error or documented as an explicit default, then confirm the TODO is replaced by deterministic behavior matching the acceptance criterion.

Written by the indexing model from the issue text.