Studio: Fix console errors on content-tab-assistant-test.tsx

[katinthehatsite]

Related to #245

Proposed Changes

This PR fixes the console.error warnings for content-tab-assistant-test.tsx that were introduced when the PR mentioned above was merged:

Although the tests are passing, the console throws errors. This happened because I introduced a new hook use-fetch-welcome-message that needed to be mocked in content-tab-assistant.

Testing Instructions

• Pull the changes from this branch
• Run npm run test
• Observe that there are no warnings coming from content-tab-assistant-test.tsx and all the tests pass
• Alternatively, you can check unit tests in Buildkite and confirm that there is no warnings once all the checks on this PR pass

Pre-merge Checklist

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

[jeroenpf]

Checks pass and the error related to content-tab-assistant-test.tsx is no longer shown.