[Content Planner] limit `description` in the `get_suggestions` response to 1000 characters

[pls78]

In the get_outline route we enforce a 1,000 characters limit for the description parameter (see here).
This parameter comes from the previous get_suggestions call, where we don't check its length to be < 1000.
We must truncate it to 1000 characters (fix here and arguably here, but please check).

See this Slack thread for more context.