Fix epic_notable_path to handle nested subgroups correctly

What does this MR do and why?

Describe in detail what your merge request does and why.

The epic_notable_path method was only extracting the first group segment from URLs, causing comments to be posted to wrong epics.

This fix:

• Extracts the full group path including all nested subgroups
• Properly handles the /-/ separator in GitLab URLs
• URL-encodes group paths for GitLab API compatibility / becomes %2F)
• Adds nil guard to prevent crashes when group path cannot be extracted

Examples:

• https://gitlab.com/groups/gitlab-org/quality/analytics/-/epics/30 → /groups/gitlab-org%2Fquality%2Fanalytics/epics/30
• https://gitlab.com/groups/gitlab-org/-/epics/23 → /groups/gitlab-org/epics/23

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review