Add a property to all features in a GeoJSON FeatureCollection.

Operates at the serde_json::Value level — no full GeoJSON DOM construction, just lightweight JSON manipulation.

Arguments

• input — GeoJSON string (FeatureCollection only).
• key — Property key to add.
• value — Property value (parsed as JSON: strings, numbers, booleans).

Returns

Modified GeoJSON string with the property added to every feature.