Make wpcom.req.* default response type unknown and add more zod schemas

[fredrikekelund]

Related issues

• Related to https://github.com/Automattic/dotcom-forge/issues/10257

Proposed Changes

Our type definitions in src/custom-package-definitions.d.ts are very lax for the Request class in the WPCOM module – it sets the default response data type to any.

Now that we have zod in the codebase, defaulting to unknown is much more fitting since this forces us to validate the response data.

This PR makes that change, and fixes a few bugs I discovered along the way:

• Our previous type definition for /studio-app/ai-assistant/chat API responses was incorrect. The topmost id prop is a number, not a string.
• This means the Message[ 'chatApiId' ] type was also invalid (previously string, now number). Same thing for the chatApiIdDict state.
• The chat Redux reducer previously stored prompt usage data in a promptUsageDict object, dividing it by site. This doesn't make sense since we limit prompt usage per WordPress.com user, not per site. I changed the state structure accordingly.

Testing Instructions

1. Smoke test the Assistant
   1.1 Ensure previous conversations load as expected
   1.2 Ensure sending new messages works as expected
   1.3 Ensure the prompt usage meter in the settings modal updates as you send messages to the Assistant

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?