chore(data-warehouse): Dont allow deleting of tables with a source

[Gilbert09]

Problem

• This isn't possible from the frontend right now, but it is possible via our frontend API and so I just wanna make a guard on the backend to make sure sourced tables dont get deleted

Changes

• Dont allow deletions of Table objects if attempted via the API

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Added tests

[greptile-apps]

PR Summary

This PR adds a guard in the TableViewSet.destroy method to prevent deletion of data warehouse tables that have an associated external data source.

• Added validation in posthog/warehouse/api/table.py that returns a 400 error if attempting to delete a table with an external data source
• Added two test cases in posthog/warehouse/api/test/test_table.py to verify both successful deletion of regular tables and blocked deletion of sourced tables
• Fixed a typo in the variable name from 'souce' to 'source' in the existing test_update_schema_400_with_source test
• This is a defensive backend measure since the frontend already prevents this action

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