fix: return 404 for unknown advisory IDs

[w3lld1]

Summary

• return no result when an advisory ID does not exist instead of propagating DoesNotExist
• add regression coverage for an unknown advisory ID

Validation

• python -m pytest -o addopts='' --nomigrations -q vulnerabilities/tests/test_same_avid_different_content_id.py (7 passed using SQLite)
• black --check vulnerabilities/models.py vulnerabilities/tests/test_same_avid_different_content_id.py
• isort --check-only vulnerabilities/models.py vulnerabilities/tests/test_same_avid_different_content_id.py
• python -m compileall -q vulnerabilities/models.py vulnerabilities/tests/test_same_avid_different_content_id.py
• git diff --check

The full PostgreSQL-backed suite is deferred to CI because PostgreSQL is not available in my local environment.

Fixes #2396