Add decodeGeohash

There is a method `geohashForLocation` which can generate geohash for [lat, lng], but there is no method for reverse

Start by locating the existing geohashForLocation method and trace how it encodes [lat, lng]. Define the expected input and decoded return value for decodeGeohash, including invalid or boundary cases, then add coverage for the agreed behavior and confirm the existing encoding path remains compatible.

Written by the indexing model from the issue text.