Behavior is the source code.

# Fetch Price

> Boundary: `fetch_price(url: string) -> number`
> Runs as: `python`

## Purpose

Provide the price value from a JSON HTTP response.

## Behavior

Fetch JSON from the given URL and return the `price` field as a
number. If the field is missing, fail with an error that mentions
`price`.

## Examples

### The service returns a price

When the URL serves a price, the chapter returns that price.

Fixture `http_fixture`:
```json
{ "price": 19.99 }
```

Returns:
```json
19.99
```

### The service omits the price

When the URL serves JSON without a price field, the chapter
rejects it.

Fixture `http_fixture`:
```json
{}
```

Fails with error containing: price