Improve display of SSVC decision tree in UI using YAML format #2058

[Mahaboobunnisa123]

Summary

Resolves #2058
This PR improves the readability of SSVC decision trees in the advisory detail UI by rendering them as clean, human-readable YAML instead of Python's default pprint repr output.

What changed

File	Change
vulnerabilities/templatetags/ssvc_filters.py	New custom Django template filter to_yaml using saneyaml
vulnerabilities/templates/advisory_detail.html	Replaced {{ ssvc.options|pprint }} with {{ ssvc.options|to_yaml }}
vulnerabilities/tests/test_ssvc_filters.py	Added tests covering normal SSVC options, empty inputs, and edge cases

Why saneyaml?

I chose saneyaml over PyYAML because it's already a project dependency (setup.cfg line 80) and the project historically prefers it (ref: #452).

Before

[{'Exploitation': 'active'}, {'Automatable': 'yes'}, {'Technical Impact': 'total'}, {'Mission Prevalence': 'essential'}, {'Public Well-being Impact': 'irreversible'}, {'Mission & Well-being': 'high'}]

After

- Exploitation: active
- Automatable: yes
- Technical Impact: total
- Mission Prevalence: essential
- Public Well-being Impact: irreversible
- Mission & Well-being: high

Tests

All 3 tests pass locally:

vulnerabilities/tests/test_ssvc_filters.py::test_to_yaml_with_ssvc_options PASSED
vulnerabilities/tests/test_ssvc_filters.py::test_to_yaml_with_empty_value PASSED
vulnerabilities/tests/test_ssvc_filters.py::test_to_yaml_with_non_serializable_value PASSED

No new dependencies were added. Happy to make any adjustments based on review feedback!