fix(#34636): replace SimpleNamespace with model instances in feedback…

[CUIRJKS]

Fixes #34636

Summary

This PR updates the feedback export integration test to use real model instances instead of SimpleNamespace test doubles.

• Replace SimpleNamespace objects with Message, Conversation, and App instances in the feedback export test
• Add the required Message fields used by the export flow, including app_id and message
• Keep the test setup aligned with production models so the export path is exercised with realistic objects

No additional dependencies are required.

Screenshots

Before	After
N/A (test-only change)	N/A (test-only change)

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods