Add tests to verify source requirement and timestamp functionality

[Copilot]

This PR adds tests to confirm that the index endpoint correctly requires the source parameter and that timestamps are properly working in the database.

Changes:

• Added a test to verify that the endpoint returns a 400 error when a request is made without the required source parameter.
• Added a test to confirm that timestamps are correctly saved in both the documents and document_chunks tables when indexing documents.
• Verified that these validations work as expected and don't break existing functionality.

The tests demonstrate that:

1. The source parameter validation is working correctly
2. Timestamps are being properly set with valid datetime values when documents are indexed

Fixes #14.

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