Add an endpoint to fetch AI task prompts for multiple workspace builds at the same time. A prompt is the value of the "AI Prompt" workspace build parameter. On main, the only way our API allows fetching workspace build parameters is by using the/workspacebuilds/$build_id/parameters endpoint, requiring a separate API call for every build.

The Tasks dashboard fetches Task workspaces in order to show them in a list, and then needs to fetch the value of theAI Prompt parameter for every task workspace (using its latest build id), requiring an additional API call for each list item. This endpoint will allow the dashboard to make just 2 calls to render the list: one to fetch task workspaces, the other to fetch prompts.

Related tocoder/internal#660.