test : added unit tests for activity-formatter DiscussionCommentEvent

[tmdeveloper007]

Refs #1280.

Summary of What Has Been Done:
Added unit tests for activity-formatter covering DiscussionCommentEvent and edge cases with missing payload fields.

Changes Made:

• Enhanced test/activity-formatter.test.ts with 5 new test cases:
  • DiscussionCommentEvent formatting
  • Missing payload fields in PushEvent
  • Empty repo name handling
  • DiscussionEvent with missing fields
  • ReleaseEvent with missing fields

Impact it Made:
Improved test coverage for activity event formatting. All 6 tests pass.

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GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus