Fix: Standardize User-Agent headers across Importers (#2122)

[NamanmeetSingh]

Fixes #2122

Description:
This PR implements a standardized User-Agent header across all network-calling modules to prevent upstream bot-blocking (like 403 Forbidden errors), as outlined in Issue #2122.

Note: @Kiran95021 had a great write-up for this issue previously! Since there hadn't been any recent activity on it and GSoC prep is ramping up, I went ahead and drafted the implementation to help get this unblocked. If you were already working on this locally, please let me know—I'm happy to collaborate!

Changes Made:

• Added a VC_USER_AGENT constant to vulnerablecode/settings.py.
• Updated requests.get, requests.post, and urllib calls inside importers, pipelines, datasources, and utils.py to securely pass the new header.
• Injected the header into shared session objects (e.g., requests_with_5xx_retry).
• Updated the pytest mock files (using mock_get.call_args and **kwargs in monkeypatches) to strictly assert and validate the presence of the new header.

Testing:

• Ran docker compose exec vulnerablecode ./manage.py import nginx_importer locally on WSL2.
• Successfully bypassed the bot-blocks, fetched the index, and cleanly inserted 43 advisories.