Add mock-based unit tests for osv_source.py

[NeilMathew-git]

This PR adds mock-based unit tests for osv_source.py to improve test coverage without relying on real API calls or external data. It uses AsyncMock and simulated file content to test key OSV behaviors in isolation.

[JigyasuRajput]

hey! @NeilMathew-git,
thanks for looking into this, can you please double check the changes as currently it shows "0 lines changes", also it's generally a good practice to mention the issue you're working on!

[NeilMathew-git]

Hey! Thanks for the heads up — looks like I might’ve missed committing the changes properly. I’ll double-check my local branch and push the correct changes shortly. Also noted on mentioning the issue — I’ll be sure to edit it in the description. Appreciate the feedback!

[terriko]

I see you've already got good feedback above, so I'm just marking this as needing changes.