test : added edge case tests for ai-prompts weeklyProductivityPrompt

[tmdeveloper007]

Refs #1276.

Summary of What Has Been Done:
Added edge case unit tests for the weeklyProductivityPrompt function in src/lib/ai-prompts.ts, covering boundary values and unusual inputs.

Changes Made:

• Enhanced test/ai-prompts.test.ts with 4 new test cases:
  • Zero values (activeDays: 0, totalCommits: 0, prsMerged: 0)
  • Large numbers (totalCommits: 99999, prsMerged: 500)
  • Negative streak values
  • Empty string for topRepoName

Impact it Made:
Improved test coverage for AI prompt generation with edge case handling. All 5 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