Remove hardcoded App Service logs link from error message

[pamelafox]

This PR removes the hardcoded aka.ms/appservice-logs link from the generic error message, since the app can run on either App Service or Container Apps.

Changes:

• app/backend/error.py: replace link with deployment-agnostic guidance: 'check the application logs for a full traceback.'
• Updated tests and snapshots that assert the exact error message.

All tests pass locally.

Fixes #2678.